feat: add string manipulation functions derived from musl libc

This commit introduces a variety of string manipulation functions, including:
- `memmem`: Searches for a substring in a memory area.
- `memmove`: Safely copies memory areas, handling overlaps.
- `mempcpy`: Copies memory and returns a pointer to the end of the destination.
- `memrchr`: Finds the last occurrence of a character in a memory area.
- `memset`: Fills a memory area with a specified value.
- `rindex`: Returns the last occurrence of a character in a string.
- `stpcpy` and `stpncpy`: Copies strings and returns a pointer to the end.
- `strcat`, `strchr`, `strcspn`, `strlcat`, `strlcpy`, `strlen`, `strncat`, `strncmp`, `strncpy`, `strnlen`, `strpbrk`, `strrchr`, `strstr`, `strtok`, and `strtok_r`: Various string manipulation functions for concatenation, searching, and tokenization.

Assembly implementations for `memcpy`, `memmove`, and `memset` are also included for optimized performance on x86_64 architectures.

1 files changed, 16 insertions, 0 deletions

diff --git a/na/lib/string/x86_64/memmove.s b/na/lib/string/x86_64/memmove.s
new file mode 100644
index 0000000..172c025
--- /dev/null
+++ b/na/lib/string/x86_64/memmove.s
@@ -0,0 +1,16 @@
+.global memmove
+.type memmove,@function
+memmove:
+	mov %rdi,%rax
+	sub %rsi,%rax
+	cmp %rdx,%rax
+.hidden __memcpy_fwd
+	jae __memcpy_fwd
+	mov %rdx,%rcx
+	lea -1(%rdi,%rdx),%rdi
+	lea -1(%rsi,%rdx),%rsi
+	std
+	rep movsb
+	cld
+	lea 1(%rdi),%rax
+	ret