Add support for expression trees (#58)

[andryak]

Summary

This diff adds support for module level attributes, such as the following:

<?hh

<<file: SomeClass(SomeOtherClass::class)>>

Requirements

• [x] I've added tests for any new code and ran npm run test-corpus to make sure all tests pass.
• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

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