parser support for parameter types ($T)
fix debug line.
add == and != to string.
add typecheck support for type parameters.

i can't believe that works. when typechecking a call, make canCoerceFrom() allow any type to fit a type parameter ("$A"), and track the alias in a map. if the overall coercion works, use the map to "fill in the blanks" of the return type. also clean up disjoint type merging a bit, since we need to try to merge after filling in the blanks too.
fix docs/ast for types.
remove dead code.
fix recursive coercion of structs.