Implement attributes caching

[stephannv]

Before (~5x slower):

Blueprint::HTML 1.0.0 937.83k (  1.07µs) (± 0.45%)  2.69kB/op   5.61× slower
                  ECR   5.26M (189.98ns) (± 0.22%)  1.51kB/op        fastest

After:
Single thread (~3x slower):

Blueprint::HTML 1.0.0   1.42M (704.57ns) (± 0.56%)  2.69kB/op   3.67× slower
                  ECR   5.20M (192.24ns) (± 0.70%)  1.51kB/op        fastest

4 threads (~2x slower):

Blueprint::HTML 1.0.0 801.36k (  1.25µs) (± 1.36%)  2.69kB/op   2.35× slower
                  ECR   1.88M (530.54ns) (± 1.18%)  1.51kB/op        fastest

[matthewmcgarvey]

This will still have the same bloating problem, but the speed is great!