Lookahead buffer for codecs

Since some codecs are multibyte, a lookahead may be required to store the next bytes and consume them only if a valid character is read.

Property definitions

core $ Stream :: lookahead=
	# Lookahead buffer for codecs
	#
	# Since some codecs are multibyte, a lookahead may be required
	# to store the next bytes and consume them only if a valid character
	# is read.
	protected var lookahead: CString is noinit
lib/core/stream.nit:36,2--41,43