If we return `Self, Self::Err` then the implementations of `FromStr` are
easier to maintain.

As is customary put the associated type at the start of the `FromStr`
impl block.

In preparation for adding separate errors move the hex code to its own
private module.

Add a wrapper type `PreallocatedBuffer` that maintains the invariant
that the inner buffer is big enough. This allows us to take this as a
parameter for preallocated context constructors and remove the error
path.

Using `TryFrom` is more terse with no loss of clarity.

Refactor only, no logic changes.

Currently we have a large general error type.

Create specific error types for each function as needed so that a
function returns ever variant available in its return type.