[fit] Allow fit::function to participate in overload resolution

Previously the fit::internal::function constructor accepted an
unrestricted template type, allowing it to match anything even if it
would later fail to compile in a way not detected by SFINAE. Adding a
restriction to the template to allow implicit fit::function casts to
participate properly in overload resolution.

Test: added unit test
Change-Id: I45d937a6792eece5ddfea8d235df81029b53aebf

[fit] Add a mechanism for sharing functions.

This implementation is somewhat inefficient but will help port
some existing std::function based code to fit::function.
We may refactor this in the future based on that experience.

Change-Id: Ic2fde955fdc145fcc8be9503a7d1041e084d7064

[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