Mojo1.0.0b1 standardizes on 'def', enforces bounds checking

Mojo 1.0.0b1 now requires 'def' to declare all functions, marking 'fn' as deprecated and warning on its use.

Bounds checking is enforced by default on all standard library collections, and negative indexing has been removed. Stateless closures are now auto-lifted to top-level functions and usable as FFI callbacks.

The release also fixes numerous compiler, runtime, formatting, and debugging issues.