Speedup: Validation of macros-only Manifest in parse_node is very time-consuming

[cmcarthur]

@jthandy noticed that on a project with hundreds of models and hundreds of tests, parsing alone can take a long time. I asked him to profile the project in question, and it turns out that around 30-35 seconds is spent in parsing nodes alone. A very large component of that parsing is building and validating the macros-only Manifest in BaseParser.parse_node (at time of writing parsers/base.py line 107).

Profile result indicates each Manifest.__init__ call takes about 45ms. The vast majority (>90%) of that is validation. In a project with 750 models + tests, that adds up to about 34 seconds added to a run that we could strip off.

I can imagine some potential fixes:

• we could cache the macros-only Manifest that gets re-used in parse_node
• we could change parse_node to use the jinja AST to pull out refs and config values
• we could skip validation when building the "fake" macros-only Manifest

Any one of these would result in an enormous speedup for the project in question.