0.19.2

• Generic type variables no longer need to be single-letter names (for example class Gen(Foo) is now possible)
• Added syntax to denote free variables: def foo(x : T) forall T. The old rule of single-letter name still applies but will be removed in the future.
• Removed the restriction that top-level types and constants can't have single-letter names
• Added @[Extern] attribute to mark regular Crystal structs as being able to be used in C bindings
• Faster Char#to_s when it's ASCII: this improves the performance of JSON and CSV parsing
• crystal spec: allow passing --release and other options
• crystal spec: allow running all specs in a given directory
• crystal playground: support custom workbook resources (thanks @bcardiff)
• crystal playground: standard output now understands ANSI colors (thanks @bcardiff)
• Added many more macro methods to traverse AST nodes (thanks @BlaXpirit)
• Error messages no longer include a type trace by default, pass --error-trace to show the full trace (the trace is often useless and makes it harder to understand error messages)
• Several bug fixes