Lukas's latest PR actually removed this custom `TypeMap` in favour of  a straight `restpe.substSyms(skolems, tparams)`. Which got me to thinking about how this version might be expressed in the same way; which other custom type maps could be expressed in the same way, etc.

What I'm thinking of is `dealiasLocals(method, restpe)..substSyms(skolems, tparams)`, where `dealiasLocals` would dealias any types aliases that are local to the method. Does that sound like a reasonable approach? How would I build such a thing?