Extend assignment function notation / make more things expressions

I think it would be useful if "match" and "case" statements were actually expressions. For example, I'd like to be able to do things like:

```
def whats_this(foo) =
case foo:
match 0:
#
"Zero"
match 1:
#
"One"
else:
"Dunno"

assert "Zero" == whats_this(0)
assert "One" == whats_this(1)
assert "Dunno" == whats_this(2)
```

The issue names no files or tests; start by tracing Coconut's compiler and parser entry points for match and case. Use the example as the acceptance case: assignment-function notation should accept these constructs as expressions and return the selected branch value, including the else branch.

Written by the indexing model from the issue text.