compiler: Use compact representation for SourceText

SourceText is serialized along with INLINE pragmas into interface files. Many of
these SourceTexts are identical, for example "{-# INLINE#". When deserialized,
each such SourceText was previously expanded out into a [Char], which is highly
wasteful of memory, and each such instance of the text would allocate an
independent list with its contents as deserializing breaks any sharing that might
have existed.

Instead, we use a `FastString` to represent these, so that each instance unique
text will be interned and stored in a memory efficient manner.