P0448R4 `<spanstream>`

[StephanTLavavej]

P0448R4 <spanstream>

Feature-test macro:
#define __cpp_lib_spanstream 202106L

@CaseyCarter's internal VS-PR-339694 has taught the IDE to recognize this as a C++ header.

[miscco]

FYI I am working on this, need more tests though