Improve perf of From<CompactString> for String in edge case

[ParkMyCar]

Originally implemented in #118, it might be possible to improve the performance of the edge case when we have a CompactString > 16MB on 32-bit systems by re-using the already allocated buffer.

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[CAD97]

Copying over my notes:

the layouts [of String's buffer and our heap allocated len+data] aren't compatible

Yeah, you'll run into an issue as String's buffer is u8 aligned but the inline length is usize aligned. At a performance cost, we could store the length unaligned to make the allocated alignment 1.

The unstable Allocator trait allows requesting (though an implementation can theoretically refuse to fulfill) a change in alignment of an allocation. The stable GlobalAlloc does not.

My vote is to just copy/realloc, and put a note tl if/when Allocator is stabilized optimize it to opportunistically use a layout-adjusting realloc so the underlying allocator can avoid the copy.