This PR fixes name mangling to be unambiguous / injective by adding `00`
for disambiguation where necessary. Additionally, the inverse function,
`Lean.Name.unmangle` has been added which can be used to unmangle a
mangled identifier. This unmangler has been added to demonstrate the
injectivity but also to allow unmangling identifiers e.g. for debugging
purposes.
Closes#10724
Preparation for new unifier.
@dselsam: I had to make minor changes at `Synth.Lean`, and add messy
code to `Context.lean`. `Context.lean` will be deleted in the future.
So, it is not a big deal.
