feat(stdlib): add Slice.hashData() and String.hashData()

[novusnota]

A follow-up to #1936, where we've fixed the sha256() for Slices and that warranted introduction of gas-efficient functions for hashing only the data of a given Slice/String.

Issue

Closes #1950.
Closes #1963 (small typo).

Checklist

• I have updated CHANGELOG.md
• I have documented my contribution in docs/ and made the build locally
• I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• I have run all the tests locally and no test failure was reported
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings