[fit] Adding FunctionTraits

This is a utility that captures elements of interest from a
function-like type.

MI4-1277 #comment

Test: unit and compile-time tests
It looks like fit-test is not deployed to zircon right now, but
verified that the compile-time tests work.
Change-Id: I2cb5eba9e20bc363e5c34153a7b4c636b0313d5e