Patch up and add tests on #2112

This replaces all deprecated function usage by alternative ones based on
a version switch enacted at compile time, preventing all warnings.

This will likely introduce some possible runtime errors in using a
Rebar3 compiled on OTP-20 or OTP-21 back in versions 19 and earlier, but
we can't really work around that.

A bunch of dependencies have been updated to support OTP-21 without
warnings as well.

This is done through 3 main change groups:

- replacing `~s` by `~ts` in format strings, so that strings that
  contain unicode are properly printed rather than crashing
- adding the `unicode` argument to all function of the `re` module to
  ensure transformations on strings containing unicode data are valid
  instead of crashing (see issue #1302)
- replacing `ec_cnv:to_binary/1` and `ec_cnv:to_list/1` with matching
  functions in `rebar_utils`.

The last point has been done, rather than modifying and updating erlware
commons, because binary and list conversions can be a contentious
subject. For example, if what is being handled is actually bytes from a
given binary stream, then forcing a byte-oriented interpretation of the
data can corrupt it. As such, it does not appear safe to modify erlware
commons' conversion functions since it may not be safe for all its
users.

Instead, rebar3 reimplements a subset of them (only converting
atoms and chardata, ignoring numbers) with the explicit purpose of
handling unicode string data.

Tests were left as unchanged as possible. This may impact the ability to
run rebar3's own suites in a unicode path, but respects a principle of
least change for such a large patch.

- Crashes in providers lib when no providers in a namespace are bare
- Making sure bareness matches semantics; i.e. a bare provider is
  visible, a non-bare provider is hidden.

- Solve "Uncaught error" failure in case of syntax error in app.src file.
- Print helpful information on the location of the syntax error.

Also expose it in rebar_api