Merge pull request deuill#16 from deuill/feature/fix-context-eval-scope

 Context: Fix variable scoping issues for `Eval`

Fix double-free and ZTS-related issues

Building against a ZTS-enabled build of PHP uncovered several issues,
including:

  * Use of missing `tsrm_ls` value. Added calls to TSRMLS_FETCH() where
    needed.
  * String keys returned with `value_array_keys` are now duplicated,
    and as such are correctly passed (and freed) to the caller.
  * Functions `value_array_index_get` and `value_array_key_get`
    incorrectly returned references of, rather than copies to, the
    values required.
  * Method `receiverSet` incorrectly called `Destroy` on the member
    value.