(RFC) stdio: consolidate the two memstream implementations

[kevans]

Summary

Substantial parts of the memstream and wmemstream implementations
overlap, and it's a little laborious to duplicate code between the two.

Abstract the differences out into some streamcfg structs and see if we
can get the arithmetic right and converge on a single implementation.

[des]

lib/libc/stdio/open_memstream.c

97

is newoff here in bytes or in characters? I think the code might be easier to reason about if everything is in characters.

[des]

lib/libc/stdio/open_memstream.c

47

See NLINK_MAX in D57324

[des]

lib/libc/stdio/open_memstream.c

47

This isn't even needed if we use ckd_add() in the SEEK_END case in memstream_seek().