Inline most of the code paths for conversions with boxed slices

This helps with the specific problem described in #49541, obviously without
making any large change to how inlining works in the general case.

Everything involved in the conversions is made `#[inline]`, except for the
`<Vec<T>>::into_boxed_slice` entry point which is made `#[inline(always)]`
after checking that duplicating the function mentioned in the issue prevented
its inlining if I only annotate it with `#[inline]`.

For the record, that function was:

```rust
pub fn foo() -> Box<[u8]> {
    vec![0].into_boxed_slice()
}
```

To help the inliner's job, we also hoist a `self.capacity() != self.len` check
in `<Vec<T>>::shrink_to_fit` and mark it as `#[inline]` too.

src/liballoc/str.rs

@@ -1811,6 +1811,7 @@ impl str {
     /// assert_eq!(*boxed_bytes, *s.as_bytes());
     /// ```
     #[stable(feature = "str_box_extras", since = "1.20.0")]
+    #[inline]
     pub fn into_boxed_bytes(self: Box<str>) -> Box<[u8]> {
         self.into()
     }
@@ -2049,6 +2050,7 @@ impl str {
     /// assert_eq!(boxed_str.into_string(), string);
     /// ```
     #[stable(feature = "box_str", since = "1.4.0")]
+    #[inline]
     pub fn into_string(self: Box<str>) -> String {
         let slice = Box::<[u8]>::from(self);
         unsafe { String::from_utf8_unchecked(slice.into_vec()) }
@@ -2307,6 +2309,7 @@ impl str {
 /// assert_eq!("☺", &*smile);
 /// ```
 #[stable(feature = "str_box_extras", since = "1.20.0")]
+#[inline]
 pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
     Box::from_raw(Box::into_raw(v) as *mut str)
 }