Update String moduledoc

`Kernel.binary_to_term` does not exist anymore

[ci skip]
Merge pull request #2283 from edgurgel/patch-1

Update String moduledoc
Properly handle atom keys in kw_list_to_string
Merge pull request #2282 from bitwalker/macro-to-string-fixes

Properly handle atom keys in kw_list_to_string
Fix race condition on agent tests
Disallow variables on the right side of in
Prepare elixir_try to be able to handle upcoming map clauses
Convert exceptions to structs
Reduce bootstrap list
Remove dependency on Record.Backend
Deprecate defrecord/3
Add Atom.to_string, Atom.to_char_list and BitString.to_list
Fix false positives when inspecting records, closes #2287