Implement Str.dropPrefix and Str.dropSuffix builtins

[lukewilliamboswell]

Implements the builtins from #664

Note I changed the api from what was described in the issue. This was easier to implement and test, and I think will be nicer to use.

• dropPrefix : Str, Str -> Result Str {} to dropPrefix : Str, Str -> Str
• dropSuffix : Str, Str -> Result Str {} to dropSuffix : Str, Str -> Str

Also, I am not 100% around the use of substringUnsafe here. I think it's ok because we are starting with a valid utf8 string, and using a valid utf8 string to drop so I don't think there is anyway to drop and now have invalid utf8. Is there a way to test this? maybe we should do some fuzzing here?

[lukewilliamboswell]

Useful information Re utf-8 validation https://roc.zulipchat.com/#narrow/stream/304641-ideas/topic/builtins.20for.20parsing/near/463098204

Thank you @timotree3

[timotree3]

Isn't this subtraction guaranteed to never exercise the saturating behavior since start = Str.countUtf8Bytes prefix <= Str.countUtf8Bytes haystack since Str.startsWith haystack prefix? It would probably be more performant to use wrapping subtraction.

[timotree3]

Stray comment

Suggested change

# = RocStr, start, len -> RocStr

[timotree3]

Also, I am not 100% around the use of substringUnsafe here. I think it's ok because we are starting with a valid utf8 string, and using a valid utf8 string to drop so I don't think there is anyway to drop and now have invalid utf8. Is there a way to test this? maybe we should do some fuzzing here?

I agree with your reasoning as long as Str.startsWith haystack prefix == Bool.true guarantees that haystack starts with the same bytes as prefix. If it was weakened to something like "haystack starts with characters that are the same as prefix after some kind of normalization" then this implementation would become buggy.

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