Erased routing, codegen opts

[zakstucke]

There's a few things in here (sorry!), to sum up:

• Routing is erased, most importantly this should (probably) solve any remaining compile errors in rust debug/dev builds
• The view!{} macro now erases thoroughly
• A few more hot codegen optimisations of existing logic in traits
• erased containers define their fn() functions with concrete fn foo declarations rather than closures, TIL this is more codegen efficient
• The erased crate is used as a replacement for Box<dyn Any> in erased containers, which is more efficient and (I think) avoids a massive dyn Any vtable
• IntoFragment contains StaticVec instead of a Vec

This shouldn't have any breaking changes.

[zakstucke]

This is almost finished, there's a second part that somehow breaks leptos, which I need to fix, but I've found the change that breaks it (not in here), I'll fix it then finish this in like 8 hrs
But after this PR, there's an erase internals of Either PR, which I'll only open if it actually benchmarks gains after this one, then the erasure journey's complete I think

[zakstucke]

Should be ready to go, updated description.

[zakstucke]

Happy to say my Either enum internal erasure PoC, on top of everything else, only provides a minimal codegen optimisation (~1%), pretty much because everything it holds is already erased, so is not worth the breaking change and weirdness of a variant that is actually a function (to handle vari-erasure) etc.

This means (I think) the erasure journey is now complete 🎉

[benwis]

This looks good to me, I'll let @gbj take a look before we merge this. Super excited!