Add missing functions to `string.h`

src/header/string/mod.rs

tests/Makefile

tests/expected/bins_dynamic/string/stpcpy.stderr

tests/expected/bins_dynamic/string/stpcpy.stdout

tests/expected/bins_dynamic/string/stpncpy.stderr

tests/expected/bins_dynamic/string/stpncpy.stdout

tests/expected/bins_dynamic/string/strchrnul.stderr

tests/expected/bins_dynamic/string/strchrnul.stdout

tests/expected/bins_dynamic/string/strsep.stderr

tests/expected/bins_dynamic/string/strsep.stdout

tests/expected/bins_static/string/stpcpy.stderr

tests/expected/bins_static/string/stpcpy.stdout

tests/expected/bins_static/string/stpncpy.stderr

tests/expected/bins_static/string/stpncpy.stdout

tests/expected/bins_static/string/strchrnul.stderr

tests/expected/bins_static/string/strchrnul.stdout

tests/expected/bins_static/string/strsep.stderr

tests/expected/bins_static/string/strsep.stdout

tests/string/stpcpy.c

tests/string/stpncpy.c