Summary:
InlineFunctionRef is a semantically the same as folly::FunctionRef but has the
additional benefit of being able to store the function it was instantiated
with inline in a buffer of the given capacity.  If there is not enough in-situ
capacity for the callable, this has the same semantics as FunctionRef.

This helps give a perf boost in the case where the data gets separated from
the point of invocation.  If, for example, at the point of invocation, the
InlineFunctionRef object is not cached, a remote memory/cache read might be
required to invoke the original callable.  Customizable inline storage helps
tune storage so we can store a type-erased callable with better performance
and locality.  A real-life example of this might be a folly::FunctionRef with
a function pointer.  The folly::FunctionRef would point to the function
pointer object in a remote location.  This causes a double-indirection at the
point of invocation, and if that memory is dirty, or not cached, it would
cause additional cache misses.  On the other hand with InlineFunctionRef,
inline storage would store the value of the function pointer, avoiding the
need to do a remote lookup to fetch the value of the function pointer.

To prevent misuse, InlineFunctionRef disallows construction from an lvalue
callable.  This is to prevent usage where a user relies on the callable's
state after invocation through InlineFunctionRef.  This has the potential to
copy the callable into inline storage when the callable is small, so we might
not use the same function when invoking, but rather a copy of it.

Also note that InlineFunctionRef will always invoke the const qualified
version of the call operator for any callable that is passed.  Regardless of
whether it has a non-const version.  This is done to enforce the logical
constraint of function state being immutable.

This class is always trivially-copyable (and therefore
trivially-destructible), making it suitable for use in a union without
requiring manual destruction.

Reviewed By: yfeldblum, ot

Differential Revision: D14029799

fbshipit-source-id: 2cff3ce27d564f3d524095189f847c14911f9402