Clean up the expander a bit

[sjamaan]

Currently the expander is full of hooks and hacks, mostly for maintaining the line number database and various syntactic environments. It would be nice if we can clean things up a bit. A few ideas:

• ##sys#syntax-context and ##sys#syntax-error/context are weird and can maybe be cleaned up if we can make ##sys#check-syntax work better with line number info.
• ##sys#syntax-error-culprit seems to be unnecessary, because while expanding ##sys#check-syntax should have access to the input form already?
• ##sys#line-number-database is defined in expander, but only really used in core.scm (except for getting the line number)
• The new expansion-result-hook is yet another hook, and can perhaps be combined with the previous two.
• The special-cased hack in ##sys#canonicalize-body that prevents expansion of import and hands back control to the compiler after expansion of ##core#module) (in a currently pending patch) needs to die. No clue yet how to do this...

[sjamaan]

Getting ready for 5.1, moving tickets which won't make it in to 5.2.