ConvertExact → ConvExact, Convert → ConvTo

[dhardy]

The major change of this PR is to remove the trait generic::Convert; this was already redundant with ConvTo so all impls are moved there.

Note: the name ConvTo is preferred over Convert for consistency

Also changed:

• Renamed ConvertExact → ConvExact (also adjusted method names) and moved to mod traits
• Reverse order of generic parameters used by CastTo, ConvTo: the R: Rounding parameter now comes first

Further note: doc changes here are minimal; more will land later.

[dhardy]

The generic parameters to ConvTo were reversed for consistency with the method calls. Unfortunately this causes an issue with trait impl orphan rules:

error[E0210]: type parameter `R` must be covered by another type when it appears before the first local type (`geom::Coord`)
   --> crates/kas-core/src/geom.rs:263:6
    |
263 | impl<R: Rounding> ConvTo<R, Coord> for kas_text::Vec2 {
    |      ^ uncovered type parameter
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local,
            and no uncovered type parameters appear before that first local type
    = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`,
            where `T0` is the first and `Tn` is the last

Edit: this commit was removed from the PR.