Enable nested records.

Instead of inferring a refined type, the compiler was inferring
the class type from the $anon class, leading to type mismatches since
it was too specific.

Conversion involving nested records does not work, but that's a
different bug.
Merge pull request #52 from hubertp/topic/nested-records

Enable nested records.
R is now renamed to Rec.

Review by @gzm0
Merge pull request #50 from vjovanov/rename-rec

R is now renamed to Rec.
Fix #51: Provide nice toString for records