Bump MSRV; implement Conv for core::{num, ops} types

[dhardy]

This supports Cast on Saturating, Wrapping and all std::ops::Range* types (excluding RangeFull which has no value).

There is already (limited) support for arrays and tuples.

Besides NonZero* (see #36), this covers all std types which I think should be covered, though there are a few others which could be supported:

• Option<T>, Result<T, _>: possible, but probably better style to use .map
• Box<T>, Cow<T>: possible, but seems pretty useless

[dhardy]

Saturating was stabilised in Rust 1.74. Since that's now two years old, I'm bumping the MSRV.