[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

[fit] Add fit::defer.

fit::defer is a replacement for fbl::AutoCall and fxl::AutoCall.
It behaves the same way but it has a default constructor and it
eagerly destroys its target after it is canceled or called much
like if the target was held in an std::optional (which we don't
have in C++14 unfortunately).

Bug: ZX-2222
Test: fit-test
Change-Id: I24f0004d468377c40925288929660aa9cc3705a6

[fit] Define fit::function.

Introduce a new library, libfit, which is intended to contain a
small number of abstractions that serve as a kind of "annex" to
the C++ 14 standard library.

This code has been derived from fbl::Function with the following
changes:
- adopt an API style similar to that of std::function
- use hand-rolled vtables to improve code generation
- add missing target() member function
- drop AllocChecker overloads

Added an empty file to make the linker happy, as was previously
done for libfbl way back when it too was header-only.

Change-Id: I6412a1827e34a9ac63444a9bc07c692ec52d763e

[fit] Revert "[fit] Define fit::function."

This reverts commit 8ab9e9f111e491c2d1621155b6fc32191dfabba0.

Change-Id: I8fae8df30a03e202e982c1234d6c1becc5678488

[fit] Define fit::function.

Introduce a new library, libfit, which is intended to contain a
small number of abstractions that serve as a kind of "annex" to
the C++ 14 standard library.

This code has been derived from fbl::Function with the following
changes:
- adopt an API style similar to that of std::function
- use hand-rolled vtables to improve code generation
- add missing target() member function
- drop AllocChecker overloads

Change-Id: I2d6b401bfc60df007ca1554124f525785344d5b9