validateProps for new reconciler

[AmaranthineCodices]

Supersedes #94; closes #24. Functionally the same as #94, but for the new reconciler instead! Briefly, this adds a new static method to components:

static validateProps(props: table) -> (false, message: string) | true

Every time props are updated, validateProps will be called with the new props before shouldUpdate and init are called. validateProps returns the same parameters that assert expects: a boolean, true if the props passed validation, false if they did not, plus a message explaining why they failed. If the first return value is true, the second value is ignored.

This is intended to be used with a type checking library like PropTypes or t.

One question remains about this: should this use GlobalConfig to conditionally disable it? I know we're looking to move away from GlobalConfig, so I'm not sure!

Checklist before submitting:

• Added/updated relevant tests

[AmaranthineCodices]

New concern: is there a better/shorter name than assertCalledWithDeepEqual?

[ZoteTheMighty]

As discussed, we'd like to hide this behind a GlobalConfig flag that defaults to false.