[fidl] Add a tree visitor to the AST.

This adds a class that lets clients walk the AST, a class
that lets clients walk the AST in declaration order, and
a way of doing testing with .fidl files instead of baking
in raw strings.

Test: Tests added in visitor_unittests.cpp

Change-Id: I5d4824cdb80a8acd8a3cfb8e9cb5586bdeacf365