Shave off a field of Buffer

[Bodigrim]

text/src/Data/Text/Internal/Builder.hs

Lines 228 to 231 in 5f343f6

	data Buffer s = Buffer {-# UNPACK #-} !(A.MArray s)
	{-# UNPACK #-} !Int -- offset
	{-# UNPACK #-} !Int -- used units
	{-# UNPACK #-} !Int -- length left

Isn't the last field redundant? I imagine it can always be restored as len(buffer) - offset - used?..