Enforce values that must not be negative at a type level

[alice-i-cecile]

What problem does this solve or what need does it fill?

Some values, such as flex_grow and flex_shrink, must not be negative, but are stored as a simple f32.

Documentation is a poor way to enforce this sort of constraint in Rust.

What solution would you like?

Use a non-negative float wrapper type. We can either hand-write this. or reuse an existing crate's work if something suitable exists.

Additional context

Noticed due to #206.

We should keep an eye out for other values that are similarly bounded as part of these changes.

[nicoburns]

Possibly shouldn't have listed this ticket in the PR description. This is NOT fixed by #269