Fix parsing context for lambdas and modules

[Morriar]

Motivation

This pull-request fixes the context used when entering classes, modules and lambdas (port of whitequark/parser#709).

This fixes a bug where the parser rejected valid code:

class A; ->(a = (return; 1)) {}; end
#                ^^^^^^ error: invalid return in class/module body

Which is now correctly parsed.

Test plan

See included automated tests.

[jez]

This mostly looks good to me. I'm happy to merge after you have a chance to answer my questions.

[Morriar]

Let's merge this one as this and complete the work in #3344.