Compiling in parallel only bought us 2s (out of 24s) since
compiling graphemes is what takes most of the time but it
considerably increases memory usage, by about 30%, since both
compilation targets are loaded into memory at once.
Based on a suggestion @krestenkrab and an initial implementation
proposed in #1902, we can reduce the total size of the String.Graphemes
beam file by ensuring the function clauses for next_grapheme are
similar.
We have done this by implementing next_grapheme in a tail recursive
fashion and using binary_part to select the first bits. The end result
is a consistently 33% faster graphemes operation for ascii strings
and only 10% slower operation for heavy unicode strings (which may
be further optimazable).
José Valim