Define midpoint and lerp functions for C++20 (P0811R3)

Define midpoint and lerp functions for C++20 (P0811R3)

The implementation of midpoint used for integral types is due to Howard
Hinnant and avoids a branch for int and larger types (but not for chars
and shorts).

The midpoint and lerp functions for floating point types come straight
from the P0811R3 proposal, with no attempt at optimization.

	* include/c_compatibility/math.h [C++20] (lerp): Add using
	declaration.
	* include/c_global/cmath [C++20] (__cpp_lib_interpolate): Define.
	(__lerp): Define function template to implement lerp.
	(lerp(float, float, float), lerp(double, double, double))
	(lerp(long double, long double, long double)): Define for C++20.
	* include/std/numeric [C++20] (__cpp_lib_interpolate): Define.
	(midpoint(T, T), midpoint(T*, T*)): Define.
	* include/std::version [C++20] (__cpp_lib_interpolate): Define.
	* testsuite/26_numerics/lerp.cc: New test.
	* testsuite/26_numerics/midpoint/floating.cc: New test.
	* testsuite/26_numerics/midpoint/integral.cc: New test.
	* testsuite/26_numerics/midpoint/pointer.cc: New test.

From-SVN: r269398