String dump with correct escaping

[comodoro]

This pull request is not complete and there are things that require clarification:

• It passes tests except for test_bug_148 which I do not understand since it seems to test exactly opposite behavior to the one in the issue

• It may be easy to add an option to dump to multiline as well as escaping or not escaping all Unicode

When complete should fix #201 and indeed #148, related to #236

Reference used: https://github.com/toml-lang/toml#user-content-string

[roedoejet]

Any reason why this hasn't been merged already?

[comodoro]

As far as I remember, it is not complete for all cases and this PR explores the maintainer's attitude. It was started because of issues that surfaced in the Caster project and was not worked upon because of no response here which resulted in a switch to tomlkit.