distinguish instance attribute models from class attribute models

Summary:
In Python, the semantics of class and instance variables differ. If you declare `class C: x = 1` and write `self.x = 2` later, `C.x` will remain unchanged for the other instances of `C`.

Previously, we would allow accessing an attribute model through both the instance and class. This disallows us from expressing models only for class attributes vs. only for instance attributes.

Support the class attribute case via the module_name.ClassName.__class__.attribute, and keep instance attribute syntax as is (via module_name.ClassName.attribute). If we need models for both cases, we'll need to write both.

Reviewed By: fahndrich

Differential Revision: D16672643

fbshipit-source-id: 92ff7bcf0863514ab5da670ed6f9e8474e78799e

Remove special treatment for `@contextmanager` and `@asynccontextmana…

…ger`

Summary:
We used to hard-code the behavior of `contextmanager` and `asynccontextmanager`. Problems are:
- With D15011034 adding support for user-defined decorators we no longer need to special-case them.
- Hard coding class names lead to sadness when the hardcoded name [changes in typeshed](python/typeshed@668d050)

I also pulled the relevant classes from the helper stubs into `contextlib` stubs and replace them with the actual typeshed stubs.

Reviewed By: shannonzhu, mrkmndz

Differential Revision: D16166530

fbshipit-source-id: 9d5497dde51724b6e8772cef119e5bd4609f9d79

Adding first draft of subtyping rules

Summary:
This is a first draft at documenting the main component of the subtyping rule:
`less_or_equal`. We still need `join` and `meet` to have a realistic package.

The style is to write down the rules in the same order as the code. At some
point we might want to change to a style with only "positive rules" (e.g. don't
write anything about Top <= T, because if there is no rule to build it, we
can't prove it). That might evolve in the future

Reviewed By: dkgi

Differential Revision: D16120038

fbshipit-source-id: ddc1ee5df934771e1d3185bf98f3cae04e4f292d

nest `check_and_correct_annotation` where used

Reviewed By: sinancepel

Differential Revision: D15403382

fbshipit-source-id: f8ec59603a87395fa066230b99d51f40861e9dce

search for binary under virtualenv directory correctly

Summary:
A common workflow is to create a venv, thusly:

```
python3 -m venv /tmp/pyre
/tmp/pyre/bin/pip install pyre-check
/tmp/pyre/bin/pyre start
```

Now, since `/tmp/pyre/bin` isn't in $PATH, we would miss out on the binary. Make pyre initialize and pyre smarter, and try to pick up on these binaries.

This use case is particularly pertinent in OSS development.

Reviewed By: grievejia

Differential Revision: D15178027

fbshipit-source-id: 938c9d5e82b23bad5986f51bcb90760a2a4b8156

only support opam 2.0

Summary: As mentioned in facebook#142, opam 1.2 doesn't have recent package versions, and is unable to build Pyre.

Reviewed By: grievejia

Differential Revision: D15133329

fbshipit-source-id: eef93a8b9965c7a862637e0dedcc3de532409ff4

Stop writing strings in issue_instances and trace_frames

Differential Revision: D15016189

fbshipit-source-id: 6d558bc7ee411f989ea3a60b776ed31848e3a750

Using basic type partitioning for more principled isinstance checking

Summary:
This diff extends the ideas of D14545445 to apply to more than the special case of lists, and to work in the positive case as well.
The average case we're trying to support here looks like this:
```
x: Union[int, Set[int]]
if isinstance(x, set): # the runtime disallows using a parametric type here, and pyre will correctly interpret it as Set[Any]
   # x should be known to be List[int]
else:
   # x should be known to be int
```
In theory, the positive case should be inferred to be `Intersection[previous type, asserted type]` and the negative case to be `Difference[previous type, asserted type]`.
Without supporting these types first class, we can approximate them by opening up the unions  (and implicit unions like Optional), and doing element-wise comparisons with the boundary.
In order to support types with Anys, we need to use `is_consistent_with` rather than normal `less_or_equal`.

Reviewed By: sinancepel

Differential Revision: D15060401

fbshipit-source-id: 656b63a51b62d07e21f2c1b8fc700ca15b94a739

Add new query option (1/n)

Summary: Adds a new query option regarding coverage in files, to be used in follow up diffs to build the color tool.

Reviewed By: sinancepel

Differential Revision: D14739329

fbshipit-source-id: a685b553219a5a62c771aeac8ebf00f1926bda86

Rename AccessNew to Name

Summary: Discussion on D14463837

Reviewed By: dkgi

Differential Revision: D14470295

fbshipit-source-id: e5c3e4eaa5fdb9bf91d01b108b0a768d0eee75f6