New "asserting" expression

[marcoeilers]

Adds a new expression asserting (a) in e where a is some assertion and e some expression.
Evaluating that expression first checks if a holds, and if it does, evaluates to the value of e.

I took the syntax and some tests from @dewert99's earlier PR #663 (thank you!!) but redid the rest because the parser and PAST have changed a lot since that PR was made.

Requires changes in Silicon and Carbon to support the new expression.

[JonasAlaif]

I was thinking it would also be nice to introduce a e1; e2 syntax for expressions which would desugar to let __tmp = e1 in e2. That's a bit tangential to this, but one could then also write check(a); e2 with a function check(b: Bool): ? requires b.