refactor parse decimal

[Yu-zh]

This PR refactors @strconv.parse_decimal by using pattern matching on StringView to avoid misuses of string indexing.

[peter-jerry-ye-code-review]

‼️ This code review is generated by a bot. Please verify the content before trusting it.

Looking at the git diff, here are the potential issues I've noticed:

1. In the new code, there's a typo/syntax error in the last line of parse_decimal_from_view:

d..trim()  // Uses two dots

Should be:

d.trim()   // Should use single dot for method call

2. The function doesn't have explicit handling for empty strings. While the pattern matching will catch it, it might be clearer to add an explicit check at the start:

guard not(str.length_eq(0)) else { syntax_err!() }

3. The exponent parsing could potentially overflow since there's no limit check on the exponent value. It would be safer to add a check for maximum exponent size before performing the multiplication:

exp = exp * 10 + (digit.to_int() - '0'.to_int())
// Should check if exp exceeds some maximum value

These suggestions aim to improve the robustness and clarity of the code while preventing potential runtime issues.

[coveralls]

Pull Request Test Coverage Report for Build 5054

Details

• 21 of 27 (77.78%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 85.242%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
strconv/decimal.mbt	21	27	77.78%

Totals
Change from base Build 5046:	0.06%
Covered Lines:	5239
Relevant Lines:	6146

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💛 - Coveralls