Resolve needless_as_bytes clippy lint

Summary:
```lang=text
warning: needless call to `as_bytes()`
  --> fbcode/buck2/allocative/allocative/src/key.rs:55:19
   |
55 |         while i < s.as_bytes().len() {
   |                   ^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `s.len()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_as_bytes
   = note: `-W clippy::needless-as-bytes` implied by `-W clippy::all`
   = help: to override `-W clippy::all` add `#[allow(clippy::needless_as_bytes)]`

warning: needless call to `as_bytes()`
  --> fbcode/buck2/allocative/allocative_derive/src/derive_allocative.rs:35:15
   |
35 |     while i < s.as_bytes().len() {
   |               ^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `s.len()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_as_bytes
   = note: `-W clippy::needless-as-bytes` implied by `-W clippy::all`
   = help: to override `-W clippy::all` add `#[allow(clippy::needless_as_bytes)]`

warning: needless call to `as_bytes()`
  --> fbcode/buck2/app/buck2_core/src/ascii_char_set.rs:20:20
   |
20 |         while i != chars.as_bytes().len() {
   |                    ^^^^^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `chars.len()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_as_bytes
   = note: `-W clippy::needless-as-bytes` implied by `-W clippy::all`
   = help: to override `-W clippy::all` add `#[allow(clippy::needless_as_bytes)]`

warning: needless call to `as_bytes()`
   --> fbcode/buck2/app/buck2_core/src/pattern/ascii_pattern.rs:114:40
    |
114 |                 debug_assert!(start <= s.as_bytes().len() - 1);
    |                                        ^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `s.len()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_as_bytes

warning: needless call to `as_bytes()`
   --> fbcode/buck2/app/buck2_core/src/pattern/ascii_pattern.rs:118:55
    |
118 |                     s.as_bytes().get_unchecked(start..s.as_bytes().len() - 1),
    |                                                       ^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `s.len()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_as_bytes

errors: 0; warnings: 5
```

Reviewed By: JakobDegen

Differential Revision: D72681966

fbshipit-source-id: 612a7f69d7b029b3d4cb8d1c4f431454aa036e48

Author: David Tolnay

allocative/allocative/src/key.rs

@@ -52,7 +52,7 @@ impl Key {
     const fn hash(s: &str) -> u64 {
         let mut hash = 0xcbf29ce484222325;
         let mut i = 0;
-        while i < s.as_bytes().len() {
+        while i < s.len() {
             let b = s.as_bytes()[i];
             hash ^= b as u64;
             hash = hash.wrapping_mul(0x100000001b3);

allocative/allocative_derive/src/derive_allocative.rs

@@ -32,7 +32,7 @@ use syn::Variant;
 const fn hash(s: &str) -> u64 {
     let mut hash = 0xcbf29ce484222325;
     let mut i = 0;
-    while i < s.as_bytes().len() {
+    while i < s.len() {
         let b = s.as_bytes()[i];
         hash ^= b as u64;
         hash = hash.wrapping_mul(0x100000001b3);