Efficient implementation of stimes

[Bodigrim]

Both strict and lazy bytestrings provide instance Semigroup, but rely on the default definition of Data.Semigroup.stimes. It would be better to provide an optimized definition.

For a strict bytestring, we can immediately allocate all required memory and memcpy in a loop. This is a simple extension of the implementation for (<>):

bytestring/Data/ByteString/Internal.hs

Lines 594 to 598 in 14ebbc5

	append (BS fp1 len1) (BS fp2 len2) =
	unsafeCreate (len1+len2) $ \destptr1 -> do
	let destptr2 = destptr1 `plusPtr` len1
	withForeignPtr fp1 $ \p1 -> memcpy destptr1 p1 len1
	withForeignPtr fp2 $ \p2 -> memcpy destptr2 p2 len2

For lazy bytestrings no low level manipulations needed, one can write something similar to sconcat:

bytestring/Data/ByteString/Lazy/Internal.hs

Lines 270 to 276 in 14ebbc5

	concat :: [ByteString] -> ByteString
	concat css0 = to css0
	where
	go Empty css = to css
	go (Chunk c cs) css = Chunk c (go cs css)
	to [] = Empty
	to (cs:css) = go cs css

[elikoga]

Looking into it