Upgrade length/capacity-related functions to const-fn

[bhgomes]

The following are upgraded:

• Array::len
• Array::is_empty
• Array::capacity
• Array::is_full
• Array::remaining_capacity (added one to ArrayString because it didn't exist)
• CapacityError::new

[c410-f3r]

Arrayvec::new was const but then was renamed to Arrayvec::new_const because of bad codegen (now Arrayvec::new is just a regular method).
So, I guess an assembly demonstration of before and after will improve the changes of merging. Or maybe such thing won't be necessary, all depends on @bluss :)
IMHO, all methods should be const when possible and bad codegen should be a thing fixed by MIR or LLVM.

[bhgomes]

Not sure if this is an issue in this case since these are just accessing struct fields or performing integer arithmetic, but I could be mistaken.

[bluss]

Thanks, yes certainly it would be expected that these methods can't run into the same problem that the constructor did, in particular in that case we changed the body/implementation to make it possible to make the functions const, and we don't do that here.

Thanks for making sure that arraystring is in sync with arrayvec

[lrazovic]

Will these changes be added to the next version? Since a lot of apps/crates depends on arrayvec it would be very useful to have const fn, so those methods can also be used in other calling functions, as explained in this clippy lint.

[bluss]

yes, should be a new version soon