Add very constrained inout parameters

Adds a special inout<T> which maps to inout in verilog. inout should almost never be used and is only needed for tristate outputs. Since Spade very much isn't designed with inout in mind, this is restricted to only be passable as a #[no_mangle] input and can only be passed along to other units, eventually ending up in a pure verilog module.

Author checklist

• New Diagnostics have at least one snapshot test that triggers it
• Added a line to CHANGELOG.md, if relevant