Lazy-load syntax validators on attr access

[djrobstep]

Currently this module has quite a large import time (over 0.5 sec on my machine).

This PR reduces this by lazy-loading the syntax validators lazily on attribute access.

[jkowalleck]

the syntax_parser = Lark(grammar, start=["iri", "iri_reference", "absolute_iri"], parser=RFC3987_SYNTAX_PARSER_TYPE) is still not lazy

[djrobstep]

I suppose I can move that too

[djrobstep]

Moved. It is probably possible to go further and entirely avoid loading lark upfront. But this feels like a big enough change already for one PR.

[gogasca]

+1 for Colab https://medium.com/@gogasca_/docker-container-analysis-and-optimizations-629f58073784