Include type info for only some exprs in HIE files
This commit relinquishes some some type information in .hie files in
exchange for better performance. See Trac #16233 for more on this.
Using .hie files to generate hyperlinked sources is a crucial milestone
towards Hi Haddock (the initiative to move Haddock to work over .hi
files and embed docstrings in those). Unfortunately, even after much
optimization on the Haddock side, the .hie based solution is still
considerably slower and more memory hungry than the existing implementation
- and the @.hie@ code is to blame.
This changes .hie file generation to track type information for only
a limited subset of expressions (specifically, those that might eventually
turn into hyperlinks in the Haddock's hyperlinker backend).