Allow grouping to be callable

Before groupings were not callable. Now they are.

Bump version of clap to official 3.0 release

Also, fix binaries as they were generated incorrectly before.

Display function reference values correctly

Fixed a bug where function reference values displayed as <index> instead of @<index>.
Updated the factorial example in the book to use a floor function to reduce duplication.

Modulo always returns the positive remainder

Modulo or remainder in computer languages vary in their implementation. Tortuga defaults to mathematical theory for any numerical ambiguities. Also, fixes issues with negative numbers not being interpreted correctly.

Initial release of Tortuga

This version includes a simplified grammar.
The compiler consists of a hand-written scanner and parser, along with a tree-walking interpreter.