When looking at heap-profiles of `cabal-install`, the `(:)` constructor
stands out as the most-allocated constructor on the heap.

Having to handle 10k+ package names contributes to the allocation
numbers, especially on 64bit archs where ASCII `String`s have a 24 byte
per character footprint.

This commit is a preparatory commit to pave the way for changing
`PackageName`'s internal representation to something like
`ShortByteString` (which is essentially a thin wrapper around primitive
`ByteArray#`s which themselves have have an overhead of 2 words + one
byte per ASCII character rounded up to nearest word) which would allow
to reduce the memory footprint by a full order of magnitude, as well as
reduce pointer chasing and GC overhead.