add StringView::unsafe_make

[hackwaly]

There missing a O(1) way to create StringView from String and UTF-16 positions. This PR added a StringView::unsafe_make for that.

[coveralls]

Pull Request Test Coverage Report for Build 5582

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 92.535%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
string/view.mbt	0	1	0.0%
<!--	Total:	0	1

Totals
Change from base Build 5581:	-0.01%
Covered Lines:	6012
Relevant Lines:	6497

---

💛 - Coveralls

[peter-jerry-ye]

There exists charcodes now, so the corresponding one will be named as unsafe_charcodes.