Add a __struct__ field on defstruct and ensure proper message on struct body access
Add `:only` dep option to filter on environments

mix deps.get will by default fetch all deps unless --only flag is given.
Merge pull request #2079 from ericmj/mix-dep-only

Add `:only` dep option to filter on environments
Fixing doc typos in Kernel
Merge pull request #2085 from jwarwick/special_typo

Fixing doc typos in Kernel
Improve error message for illegal use of variable in map key, closes #2083
Support :io.get_password in StringIO
Ensure struct fields are properly escaped when injected into AST
Fixed StringIO doc typos
Fixed Kernel.SpecialForms doc typos
Merge pull request #2088 from jwarwick/doc_typos

Fixing doc typos
Make the ParallelCompiler aware of missing structs

This allows modules that have recursive dependencies on structs
to be defined by having a fine grained control on the struct
definition besides the module one.
Fixed doc typos in Module
Merge pull request #2089 from jwarwick/module_docs

Fixed doc typos in Module
Merge pull request #2087 from ericmj/string-io-get-password

Support :io.get_password in StringIO
Add `Mix.Shell.prompt/1`
Do not impose reduce/3 on Dict implementations
Inline System.stacktrace/0
Compile project if a task via the CLI cannot be found
Raise error if another capture_io process already captured the device