[WIP] [stdlib] [SE-0089] Rename String.init(_:)

[moiseev]

What's in this pull request?

Continuation of the work started by @frootloops in #3212.
PR comments have been addressed.

More work needs to be performed to update all the tests to use the new String.init(describing:) instead of String.init(_:).

Besides, as UnicodeScalar.description has been changed to return the integer value of a code point, StaticString now behaves in a weird way. Here is the example of this behavior from the REPL:

(swift) let us: UnicodeScalar = "a"; us.description
// us : UnicodeScalar = "97"
// r0 : String = "5755"

Resolved bug number: (SR-1881)

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[jtbandes]

How about description?

[gribozavr]

Continuing in #3761.