Proper handling of `;` between statements.

[ehuss]

For example, it currently allows multiple expressions without ; separators which is not correct. It will need to handle the difference of expressions with/without blocks needing ;, etc.

Also consider if Stmt should include macro invocation. Currently it is inherited via expression. Why does rustc parse statement macros separately from expressions (parse_stmt_without_recovery)?

[petrochenkov]

This is an open question, especially when macros are involved.
I'll try to finish rust-lang/rust#61733 and come up with some consistent/improved rules.

[petrochenkov]

Tentatively, ; should be a part of the statement syntax and free ;s should be (empty) statements as well.