Audit escape sequences

[IwanKaramazow]

We should check that we align with JS.

[cristianoc]

This? #469

[cknitt]

There also was an unmerged PR #470 for this that could probably be resurrected?

[cristianoc]

Continued here: #575

• Needs checking that it implements the intended semantics.
• Remove dependency on Str
• Check that the compiler is happy with it (does not produce invalid escape sequences for instance), and its tests are OK with this.

[IwanKaramazow]

What's the status of this after #575 ?

[cristianoc]

Should be done.