Sylvain Henry authored Jan 18, 2019 and Marge Bot committed Mar 08, 2019

GHC represents String literals as ByteString internally for efficiency
reasons. However, until now it wasn't possible to efficiently create
large string literals with TH (e.g. to embed a file in a binary, cf #14741):
TH code had to unpack the bytes into a [Word8] that GHC then had to re-pack
into a ByteString.

This patch adds the possibility to efficiently create a "string" literal
from raw bytes. We get the following compile times for different sizes
of TH created literals:

|| Size || Before || After  || Gain ||
|| 30K  || 2.307s || 2.299  || 0%   ||
|| 3M   || 3.073s || 2.400s || 21%  ||
|| 30M  || 8.517s || 3.390s || 60%  ||

Ticket #14741 can be fixed if the original code uses this new TH feature.