Port to new proc-macro interface

[lambda-fairy]

Now that rust-lang/rust#40939 has landed, we can start using it in Maud.

Note that the new interface doesn't have a fleshed out diagnostics API yet, so error messages won't have spans in the mean time.

[lambda-fairy]

I've done most of the groundwork on the proc-macro branch. Just the control structures (@if etc.) are missing at this point., so there shouldn't be much work left from here.

• Clippy might complain about collapsible else { if .. } blocks. This will be covered in Clippy complains about else { if .. } blocks #93.

• You should only need to touch the maud_macros/src/parse.rs module. Other tasks (like moving lints and HTML escaping into their own crates) will be done in a separate ticket.

• Feel free to refer to the existing parser for inspiration.

[lambda-fairy]

I've implemented @let expressions. Had to refactor the code to not discard spans, since they contained hygiene information (whoops).

Removing the "hard" tag because the difficult work has already been done.

[lambda-fairy]

No-one else has volunteered yet so I'm doing the rest of the work myself.

[lambda-fairy]

Only thing left is @match expressions now.

[lambda-fairy]

Implemented in #95