ratelimits: combine validation and precomputation into a constructor

[jsha]

Reviewing https://github.com/letsencrypt/boulder/pull/7869/files/b7189f5c31798523fe61d1be0a40363cb9e9fa30..81b616be22710d38b2193f200f63a9c2bed86637#diff-ba4749ac861bae77ee5067820eff8d269f6ede6202108c0238a1809726d066d3R224-R236, I notice that we now have a pattern of partially initializing a struct, then validating its contents, then precomputing some internal fields:

		lim := &limit{
			burst:  v.Burst,
			count:  v.Count,
			period: v.Period,
			name:   name,
		}

		err := validateLimit(lim)
		if err != nil {
			return nil, fmt.Errorf("parsing default limit %q: %w", k, err)
		}

		lim.precompute()

This is a great use case for a constructor that returns an error:

func newLimit(name string, config LimitConfig) (*limit, error) {

That way we can reduce the chance of winding up with a partially-initialized limit object, and also reduce boilerplate a little bit.