checkers: add new checker stringsCompare

[peakle]

pr for: #1167

[quasilyte]

I would do this in 2 steps:

1. suggest simpler operations for the cases where the replacement is obvious.
2. Implement other cases as a hand-written analyzer, to avoid false positives.

[quasilyte]

@cristaloleg, what do you think?

[quasilyte]

Please, cover all cases with tests.
I don't see -1 value case.
Whether possible, all corner cases should be tested.

It's also a good practice to have "fixed" code examples in the negative tests suite.
For example, if linter suggests > operator, negative tests suite should contain the suggested code snippet, to be sure that this replacements does not trigger any other warning.

[quasilyte]

LGTM.
@cristaloleg wanna make a final review? :)