Stdio.Buffer: output_to() et al now support functions.

This reduces the need for proxy objects in higher level code
using Stdio.Buffer internally.

Also adds support for subtyped objects and multiple inheritance
of Stdio.Fd et al.
Stdio.File: Pass ::write to Stdio.Buffer::output_to() et al.

This fixes infinite recursion when using Stdio.FILE with output buffers.
Remove 7.6 compat files.
Remove 7.6 compat. Add 7.8 compat master.
Removed some useless version checks. Only relevant if you would moce the code to an older Pike binary.
Removed SSCANF_FLAG_76_COMPAT.
Test #pike 8.1
Removed sscanf_76_type_string
Ops. Didn't check in the last changes.
nomask is deprecated. Release the reserved token.
Removed f_sscanf_76 and f_sprintf_76.
Fixed warning.
Removed sprintf_76 tests.
Always hide empty modules.
Remove remaining explicit 7.6 compat.
Removed compat flags from sprintf and sscanf.
Removed environment compat and some other old leftovers.