async_do with arguments is useful for debugging a particular suite/test
with r3:break()

Example session:
Add r3:break() in recompile_when_hrl_changes test in test/rebar_compile_SUITE.erl

rebar3 shell
...
1> r3:async_do(default, ct, "--suite=test/rebar_compile_SUITE.erl --case=recompile_when_hrl_changes").
ok
Running Common Test suites...
%%% rebar_compile_SUITE:
=== BREAK ===

2> r3:resume().
ok
3> .
All 1 tests passed.

This is mostly useful for tests, where a test suite of any kind can be
interrupted halfway through so that the user can probe the running
system to see what is happening.

This is done as follows:

1. the user must call `r3:break()` in a test suite
2. the user runs the task as `r3:async_do(ct)`
3. the test holds up and the user can do whatever
4. the user calls `r3:resume()` and the test proceeds as normal

A safeguard is added so that breakpoints are only triggered in the shell
in async mode

Sample session:

    $ rebar3 shell
    ...
    1> rebar_agent:async_do(ct).
    ok
    ...
    Running Common Test suites...
    %%% rebar_alias_SUITE: .
    === BREAK ===

    2> % <do some checks>
    2> r3:resume().
    ok
    3> .....
    %%% rebar_as_SUITE: ...........
    %%% rebar_compile_SUITE: ......
    ...

This uses the very risky '$handle_undefined_function'/2 export from the
r3 and rebar_agent modules to allow meta-calls that can support plugins
and all other rebar3 extensions.

This is nasty but very tempting. Currently we only support:

- r3:do(Command)
- r3:do(Namespace, Command)

There is currently no way to pass arguments to the function such that we
can, for example, run cover analysis or tests on a subset of suites.

With the new abuse of '$handle_undefined_function'/2, we can detect the
unused commands (since they are not exported) and re-route them:

- r3:Command()
- r3:Command("--args=as a string")
- r3:Command(Namespace, "--args=as a string")

Of course, in doing so, we make it impossible to use the 'do' provider
(as in 'rebar3 do ct -c, cover') since the 'do' function is already
required for things to work.

Since the previous function had very strict guards, we can, without
conflict, add manual overrides that simulate the meta-calls fine.

Sample run:
https://gist.github.com/ferd/2c06d59c7083c146d25e4ee301de0073

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 shell agent allows to run rebar3 commands and autoload compiled
modules when that is done.