Revert "Just STLC for now"

This reverts commit 7419fa24ea18d31424e7c1df6dbdeff54f678561.
Revert "Just STLC for now"

This reverts commit 9f6ebf759cc3c73f1ffba0a0878de0344abde188.
Revert "Just STLC for now"

This reverts commit 30ecca89da12167da8066138b9be4559d45633e1.
Use implicit arguments and review the `subst` relation for System F