Sharing 0-length ByteArray#s

Motivation

Currently each 0-length ByteArray# on the heap takes 16 bytes of heap space. Yet, every copy is identical and could be shared just like we share constructors like Nothing.

Proposal

Modify newByteArray# 0 and newPinnedByteArray# 0 so that repeated calls provide the same shared MutableByteArray#.

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