Add exception formatting and colorization to iex
Don't use Regex module inside Exception, as it may not be available
Respond to review comments
Improve Error on bad_return from Application.Behaviour.Start
Make explicit is_range / is_regex are allowed in guards
Limit the allowed sigils
Remove uneeded mix fixtures
Allow operators to be captured, example: &+/2
Move from  // to \\
Deprecate usage of @behavior in favor of @behaviour
Address undefined macro if on bootstraping
Improve error messages for bad arity errors
Deprecate =~

Use String.contains?/2 when the right side is a string or
String.match?/2 when the right side is a regex.

The goal of this deprecation is to make space for the upcoming
`~` and `~=` operators.
Generalize unary operators followed by int have higher precedence
Fix list with keywords expansion

The list `[1, 2, three: :four]` now correctly expands
to `[1, 2, {:three, :four}]`.
IEx now loads the .iex.exs file instead of the .iex one
Prefix all warnings with [warning]

We expose :elixir_errors.warn/2 and :elixir_errors.warn/3
The first is to be used when we have detailed caller information,
such as from a stacktrace. The second is useful when our only
context is a line number and filename.

Note also that the test for module redfinition is performed in
helpers_test.exs rather than in warning_test.exs Perhaps this is
something that should be moved.
Remove Unix specific behaviour from File.cp/3 and File.cr_r/3

Previously those functions would behave as in their Unix counterpart
where copying behaved differently depending if the destination was
an existing directory or not. We have explicitly disallowed this
behaviour and added the proper notes to the docs.

Closes #2024.
Fix fixtures copying on mix tests
Remove unecessary lexical check in parser