Linear code emitter.

The emitter we inherited from ClojureScript emitted Clojure
expressions as JavaScript expressions.  C expressions do not support
loops, however, so we had to transform loop subexpressions into
separate functions which we could call from within the larger
expression.

This created a problem: On invocations, we now inherited the C
compiler's argument evaluation order, which is not specified in the C
standard, i.e., we got whatever the C compiler decided would be best.

The new emitter emits an assignment to a new variable for each
subexpression, which nicely solves the problem, doesn't require extra
functions (except for try, but it should also be possible to handle
this without an extra function), and is simpler.

Author: schani