make Parser polymorphic over stream type

[cardenaso11]

its plausible end users might want to supply their own text type to parse. Parser should be polymorphic over the stream type then

note that this will require something like foldM char (or foldM token in megaparsec 7). we'll implement this in steps:

• [x] switch to lazy text instead of strict text
• [ ] make Parser polymorphic over Token ~ Char
• [ ] make Parser polymorphic over Token unifying with a typeclass that represents a superset of ASCII