ir: Added Nodes MixinFunction and MixinTemplate
ir: Added Node MixinStatement
parser: Make a basic mixin template parse
test: Add template mixin test
parser: Make basic mixin function parse
test: Add function mixin test
test: Add more mixin tests
visitor: Implement prettyprinter for mixins
parser: Implement mixin statement
test: Add mixin() mixin Foo!() test
visitor: Enter/Leave MixinStatement
ir: Add a resolved field to MixinStatement
visitor: Handle MixinStatement.resolved
semantic: First stab at string mixins