Modernise code in rename/RnEnv.lhs

Removed a definition of thenM, and used do notation instead
Remove an old commented out import
Whitespace only in typecheck/TcHsSyn.lhs
Remove a "mappM = mapM" wrapper in typecheck/TcHsSyn.lhs
Remove a "returnM = return" wrapper in typecheck/TcHsSyn.lhs
Modernise some code

Use do notation rather than thenM in typecheck/TcHsSyn.lhs