Slices and Strings comparisons

[Gusarich]

This PR introduces the ability to compare both slices and strings via == operation.

For slice comparison, their hashes (calculated using HASHSU) can be compared directly.
As for strings, they are essentially slices and can be compared similarly. However, there's a consideration with strings due to their potential for various serializations (e.g., a string can be split in the middle and continued in a subsequent reference, still constituting a valid string). Upon reviewing the implementation of the String type in Tact, it was determined to be deterministic: all slices in a 'snake' representation are completely filled, except for the last one. Therefore, the same slice comparison method is applicable to strings as well.

[0kenx]

Hash is not the most gas efficient way to do this. There's a native opcode for this:

int equal_slices(slice a, slice b) asm "SDEQ";

[Skydev0h]

Hash is not the most gas efficient way to do this. There's a native opcode for this:

int equal_slices(slice a, slice b) asm "SDEQ";

SDEQ is, essentially, an abbreviation for Slice Data Equal. It does not account for refs at all. Use of hash covers both long (multi-cell) and short (single-cell) strings correctly.

[anton-trunov]

Thanks for contributing more features 👍.
Would be great to

• have some runtime tests with == and != both for strings and (some other kinds of) slices;
• add an entry to the changelog (to the Changed subsection).

Update: done