Based off a macro by @okeuday at https://github.com/erlang/otp/pull/1783

When a global plugin is used both locally and within the project, there
are cases when the rebar3 program will hard crash (killed in do_boot).
This has been traced to plugin-handling in compilation, where the same
code path may be purged twice in a row without further reloading for the
compile operation.

This of course yields the result where the code handling on the VM kills
all processes holding references to the module in memory, in this case
the rebar3 process itself.

By deduplicating the paths first, we ensure at most one purge before
reloading plugins and paths, and this prevents a hard crash.

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.

Includes improvments and function documentation for all modules (in
alphabetical order) up to rebar_core, and may have included more in
other modules as I saw fit to dig and understand more of the internals.

The wide-cast catching of undef errors would make it so any provider
calling an undefined function would be reported as a missing 'do' for
the provider.

This patch inspects the stacktrace to know if `Provider:do/1` is indeed
the missing callback, and reports the rest properly.

Breaking up initial call to parse from the ones deep inside the provider
parsing to do smarter namespace detection, added 'as' the ability to
look into these also, and cleaned up the code a whole lot that would
depend on implicit assumptions.

A side-effect is that 'do' is now valid for all namespaces, although it
can be overriden.

This adds some more convolution to command parsing, unfortunately, but
makes some cases clearer (`rebar3 as as as as` is no longer valid). It
unfortunately disallows using $REBAR_PROFILE along with `as` in a rebar
command.

Using a namespace such as 'rebar3 lfe new' will look for the 'new'
command in 'lfe' namespace without the need for a 'do' provider to be
registered in that namespace.

Manually checked that 'rebar3 as <profile> <namespace> command' works,
as well with 'default' and 'do'.

Test suite still needed.

And more general namespace support

Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands