[ty] fix bug in string annotations and clean up diagnostics

[Gankra]

This multiplication was always using only using the larger sub-ast size value, which didn't do anything unsound but made sub-string annotations become exhausted on files 32x smaller than expected (16k nodes instead of 512k nodes).

Also I decided to do a cleanup pass on the diagnostics to make them more precise and helpful.

[Gankra]

thanks to @MeGaGiGaGon for catching this!

[AlexWaygood]

nit

Suggested change

	NodeIndexError::TooNested => "too many levels of nested string annotations, remove the redundant nested quotes".to_owned(),
	NodeIndexError::OverflowedIndices => {
	"file too long for string annotations, either break up the file or don't use string annotations".to_owned()
	NodeIndexError::TooNested => "too many levels of nested string annotations; remove the redundant nested quotes".to_owned(),
	NodeIndexError::OverflowedIndices => {
	"file too long for string annotations: either break up the file or don't use string annotations".to_owned()

[Gankra]

huh, what's the logic for colon vs semicolon, to you?

[AlexWaygood]

I would usually use a semicolon for two statements that are independent and sit alongside each other, but a colon for two statements where the second directly follows from the first.

In this specific context... I can't really put a finger on why a colon feels better for me in the second case while a semicolon feels better in the first case 😆 I think either is perfectly grammatical. The second one feels slightly more imperative to me (not sure why), which I think is why I'd go for a colon there? But as I say, I think both are fine here

[AlexWaygood]

Suggested change

	"file too long for nested string annotations, remove the redundant nested quotes".to_owned()
	}
	NodeIndexError::ExhaustedSubIndices => {
	"string annotation is too long, consider introducing type aliases to simplify".to_owned()
	}
	NodeIndexError::ExhaustedSubSubIndices => {
	"nested string annotation is too long, remove the redundant nested quotes".to_owned()
	"file too long for nested string annotations; remove the redundant nested quotes".to_owned()
	}
	NodeIndexError::ExhaustedSubIndices => {
	"string annotation is too long: consider introducing type aliases to simplify".to_owned()
	}
	NodeIndexError::ExhaustedSubSubIndices => {
	"nested string annotation is too long; remove the redundant nested quotes".to_owned()