Add is_var/1 for use in typespec guards

Allows you to specify a variable without any type contraints.
Move typespec guards to end of spec

Moves precedence of `when` to be lower than `::`.
Updated mix.new success wording
Merge pull request #1874 from jwarwick/success2

Updated mix.new success wording
Ensure build per environment in umbrella children do not clash
Catch attempts to compile missing files in iex and give slightly tidier error messages
Merge pull request #1875 from pragdave/iex-c-error

Catch attempts to compile missing files in iex and give slightly tidier error messages
Fix the example for @before_compile hook
Merge pull request #1876 from alco/before-compile-doc

Fix the example for @before_compile hook
Release v0.11.2
Start v0.11.3-dev
Make IEx task with @moduledoc false
Compose streams as we go and remove tuples nesting
Improve coverage for streams with nested accs
Simplify how app files are generated and checked
Minor consistency update to Dict.values doc

Adds a period to the end of the first line in the doc, for consistency
with the rest of the module
Merge pull request #1880 from knewter/minor_doc_fix_on_dict

Minor consistency update to Dict.values doc
Ensure symlink_or_copy can override previous symlinks
Also symlink include directory
Make surround_many accept an optional seperator per-invocation, or fall back to module attribute default. Passes tests+doc_tests.