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.

opps. Infinite loop fixed.

https://github.com/erlang/rebar3/pull/1317 
In reference to with support to load erlang code atomically but load nifs non-atomically.

When an app contains one of the modules we can't afford to get upgraded
without breakage (i.e. erlware_commons, providers, getopt), skip
reloading that application from the rebar3 agent, such that it can keep
on working in subsequent calls.

Blacklisted:

- erlware_commons (broke on ec_cmd_log)
- providers (core to functionality)
- cf (useful to not explode on colors)
- cth_readable (only used for CT suites, but incompatibilities may break
  whole runs)

This tries to reduce memory usage when running `rebar3 shell` by running
the agent in the current process (and avoiding to copy state
cross-boundaries), and using frequent hibernation after each run to
force a full GC and compaction of the current process.

The old run didn't necessarily apply all profiles well, and would only
reload modules in the app file. However, since adding extra_src_dirs,
modules can be compiled without ending up in the app file; this lets
the rebar3 shell agent handle that case.

The feature may still be modified in the future.

This allows proper checking of new configurations, deps, or plugins, and
makes sure they are detected during an active shell session.

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