range checks in DynamicConverter

range checks in DynamicConverter

Summary:
Add range-checking to convertTo for small numeric types.

Internally, dynamics represent a numeric with an int64_t or a double.
When converting to a smaller numeric type, DynamicConverter uses a
static_cast. This causes some confusion (re D936940). The code now uses
folly::to, which throws a std::range_error on overflow.

While working on this I also added some light comments to the new
toDynamic section, for consistency with the original convertTo
commenting. I also renamed the internal trait is_associative_container
to is_map, since is_associative_container is looking for a mapped_type
typedef and hence excludes such associative containers as sets.

While adding the overflow tests, I also augmented the typetraits test to
include the is_map and is_range traits, which hitherto had no test
coverage.

Test Plan: build and run tests, both in dbg and opt

Reviewed By: cberner@fb.com

FB internal diff: D961605