Improved stdlib, variadic functions, default parameters

Features

• Standard Library written in Aria.
• A whole set of runtime functions.
• Type hinting on function parameters and return type.
• Default value on function parameters.
• Type lock on variables.
• Shorthand assignment operators.
• is operator for type checking.
• as operator for type conversion.

Improvements

• Out of bounds string subscript returns nil.
• Cache imports for more efficient loading.
• Simplify how imports are parsed for better treatment as expressions.

Bug Fixes

• Unresolved module properties now report an error.
• Module functions would fail on subsequent calls.