rust: alloc: add `RawVec::try_with_capacity_in()` constructor

Add the `RawVec::try_with_capacity_in()` constructor as the fallible
version of `RawVec::with_capacity_in()`.

The implementation follows the original.

The infallible constructor is implemented in terms of the private
`RawVec::allocate_in()` constructor, thus also add the private
`RawVec::try_allocate_in()` constructor following the other.

It will be used to implement `Vec::try_with_capacity{,_in}()` in
the next patch.

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>