Sealing session type subtyping relation

Seal subtyping relation for session types, to avoid breaking abstraction when applying subtyping rules and tactics

changed title from Jonas/sealing lsty le to Sealing lsty le

changed title from Sealing lsty le to Sealing session type subtyping relation

added 3 commits

• f00cbb9c - 1 commit from branch master
• 5326b524 - Sealed ltty_le part of lty_le to prevent breaking abstraction
• 2e7f360c - Fixed typo `ltty_le` -> `lsty_le`

Compare with previous version

mentioned in commit 78091b75

merged