I got fed up with looking at error messages that tell me "VERIFICATION
FAILED: !is_error()". So this commit introduces DiagnosticEngine class
whose purpose is to accumulate and print more user-friendly errors.
This will effectively allow us to use C++ code as an input for the
compiler. This would be useful for testing, since otherwise we would
have had to specify tests as a spec-like XML, which is not exactly the
most developer-friendly experience.
It simplifies ladders of BinaryOperators nodes in the function call
arguments into nice and neat FunctionCall node. Ladders initially appear
since I do not want to complicate expression parser, so it interprets
`f(a, b, c, d)` as `f "function_call_operator" (a, (b, (c, d))))`.
Currently, they are not extremely useful, but the plan is to store
all function-local state in JSSpecCompiler::Function and all
"translation unit" state in ExecutionContext.
