Improve macro compile time

[oscartbeaumont]

We should try and drop quote + syn to whatever level possible and see if we can get the macros to compile faster.

Realistically Serde uses syn internally so maybe this is negligible ...

[oscartbeaumont]

Also look at reducing the macro output like #309:

We should be able to shrink it to help the compiler. I'm not sure this will make a massive real world difference but I recon it's worth it for the increases debuggability too.

Changes:

• Flatten imports instead of using full qualified path everything. Eg. use specta::internal::construct; at top.
• Move more stuff into the macros runtime?