core: make str::split_at_unchecked() inline

This PR adds `#[inline]` to the method `str::split_at_unchecked()`.
This is done for two reasons:

1. The method is tiny, e.g. on AMD-64 (<https://godbolt.org/z/ba68fdfxn>):

   ```asm
   movq    %rdi, %rax
   subq    %rcx, %rdx
   movq    %rsi, (%rdi)
   addq    %rcx, %rsi
   movq    %rcx, 8(%rdi)
   movq    %rsi, 16(%rdi)
   movq    %rdx, 24(%rdi)
   retq
   ```

2. More importantly, inlining the method enables further automatic
   optimizations. E.g. if you split at index 3, then in the compiler
   (rustc, llvm or both) knows that this code cannot fail, and the
   panicking path is omitted in the generated code:

   ```rust
   pub fn punctuation(i: &str) -> Result<(), ()> {
       const THREE_CHARS: &[[u8; 3]] = &[*b"<<=", *b">>=", *b"...", *b"..="];

       if let Some((head, _)) = i.split_at_checked(3)
           && THREE_CHARS.contains(&head.as_bytes().try_into().unwrap())
       {
           Ok(())
       } else {
           Err(())
       }
   }
   ```

   <details>
   <summary>Without PR</summary>

   <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=0234de8158f467eebd73286f20d6e27a>

   ```asm
   playground::punctuation:
           subq    $40, %rsp
           movq    %rsi, %rdx
           movq    %rdi, %rsi
           movb    $1, %al
           cmpq    $3, %rdx
           ja      .LBB2_2
           je      .LBB2_3
   .LBB2_11:
           addq    $40, %rsp
           retq
   .LBB2_2:
           cmpb    $-64, 3(%rsi)
           jl      .LBB2_11
   .LBB2_3:
           leaq    8(%rsp), %rdi
           movl    $3, %ecx
           callq   *core::str::<impl str>::split_at_unchecked@GOTPCREL(%rip)
           movq    8(%rsp), %rcx
           movb    $1, %al
           testq   %rcx, %rcx
           je      .LBB2_11
           cmpq    $3, 16(%rsp)
           jne     .LBB2_12
           movzwl  (%rcx), %edx
           movzbl  2(%rcx), %ecx
           shll    $16, %ecx
           orl     %edx, %ecx
           cmpl    $4013115, %ecx
           jg      .LBB2_8
           cmpl    $3026478, %ecx
           je      .LBB2_10
           cmpl    $4009518, %ecx
           je      .LBB2_10
           jmp     .LBB2_11
   .LBB2_8:
           cmpl    $4013630, %ecx
           je      .LBB2_10
           cmpl    $4013116, %ecx
           jne     .LBB2_11
   .LBB2_10:
           xorl    %eax, %eax
           addq    $40, %rsp
           retq
   .LBB2_12:
           leaq    .Lanon.d98a7fbb86d10a97c24516e267466134.2(%rip), %rdi
           leaq    .Lanon.d98a7fbb86d10a97c24516e267466134.1(%rip), %rcx
           leaq    .Lanon.d98a7fbb86d10a97c24516e267466134.6(%rip), %r8
           leaq    7(%rsp), %rdx
           movl    $43, %esi
           callq   *core::result::unwrap_failed@GOTPCREL(%rip)
   ```
   </details>

   <details>
   <summary>With PR</summary>

   <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=5d4058c79ce0f6cb1a434190427d2055>

   ```asm
   playground::punctuation:
           movb    $1, %al
           cmpq    $3, %rsi
           ja      .LBB0_2
           je      .LBB0_3
   .LBB0_9:
           retq
   .LBB0_2:
           cmpb    $-64, 3(%rdi)
           jl      .LBB0_9
   .LBB0_3:
           movzwl  (%rdi), %eax
           movzbl  2(%rdi), %ecx
           shll    $16, %ecx
           orl     %eax, %ecx
           movb    $1, %al
           cmpl    $4013115, %ecx
           jg      .LBB0_6
           cmpl    $3026478, %ecx
           je      .LBB0_8
           cmpl    $4009518, %ecx
           je      .LBB0_8
           jmp     .LBB0_9
   .LBB0_6:
           cmpl    $4013630, %ecx
           je      .LBB0_8
           cmpl    $4013116, %ecx
           jne     .LBB0_9
   .LBB0_8:
           xorl    %eax, %eax
           retq
   ```
   </details>

Author: Kijewski

core/src/str/mod.rs

@@ -952,6 +952,7 @@ impl str {
     ///
     /// The caller must ensure that `mid` is a valid byte offset from the start
     /// of the string and falls on the boundary of a UTF-8 code point.
+    #[inline]
     const unsafe fn split_at_unchecked(&self, mid: usize) -> (&str, &str) {
         let len = self.len();
         let ptr = self.as_ptr();