Introduce a Units trait

[nicoburns]

Objective

Introduce an abstraction that allows "the whole of Taffy" to be generic over multiple types with only one generic parameter.

The core of this change is the introduction of:

/// Allows Taffy to abstract of the types it uses to represent strings and calc expressions.
/// In future it may be extended to abstract the scalar number type (currently f32)
pub trait Units {
    /// Trait that represents a cheaply clonable string. If you're unsure what to use here
    /// consider `Arc<str>` or `string_cache::Atom`.
    type Str: CheapCloneStr;

    /// Type representing a calc expression.
    /// If you're not using Calc then you can just use `()`
    type Calc: 'static;
}

pub struct DefaultUnits;
impl Units for DefaultUnits {
    type Str = String;
    type Calc = ();
}

Context

• CSS Grid: named lines and areas #840 made lots of Taffy (including the Style struct) generic over a string type
• The most obvious fix for Taffy 0.8 - taffy values cannot be sent between threads due to *const () #823 is to be generic over the Calc type.
• In future, we may want to be generic over the scalar numeric type (Allow Taffy to be used with f64 values where we currently f32 #332)

Feedback wanted

Is this a sensible change or am I going overboard with the generic types?

[alice-i-cecile]

This should note that it has no specific semantic value.

[nicoburns]

What do you mean by "semantic value"?

[alice-i-cecile]

Seems reasonable. I think the semantics are clearer and I don't mind the added generics.