The TIR equivalent of python's `return` statement is the
`tir::builtin::ret()` operator. Prior to this commit, this was
printed as `T.ret(value)`, and any use of `return` statement in
TVMScript produced an error while parsing.
This commit updates the TVMScript parsing to produce `T.ret(value)`
for the python `return value` statement. If syntax sugar is enabled,
the TIR `T.ret(value)` will be printed as `return value`.
Eric Lunderberg
ff3716b83a