Add Peek

Add a peek function to let the Lexer look at a character arbitrarily
far ahead in the stream without consuming anything.  We need this to
disambiguate numbers and operands of a paste operation.  For example:

def foo#8i

Without lookahead the lexer will treat '8' as a number rather than as
part of a string to be pasted to form an identifier.

llvm-svn: 142512