Refactor: `Katachi::KEY_PATTERN` because `:$key` is too strange on first impressions

[jtannas]

# config
Katachi::KEY_PREFIX = :kt_

# adding a shape without the prefix prepends it
Katachi.add_shape(:foo, 'foo')
Katachi::Shapes[:kt_foo] # => 'foo'

# adding a shape with the prefix leaves it alone
Katachi.add_shape(:kt_bar, 'bar')
Katachi::Shapes[:kt_bar] # => 'bar'

# prefix can be either a string or a symbol; lookup responds accordingly
Katachi::KEY_PREFIX = 'ref:'
Katachi.add_shape(:baz, 'baz)
Katachi::Shapes['ref:baz'] = 'baz'

# Changing the prefix "alters" the keys of all existing shapes
Katachi::Shapes['ref:foo'] = 'foo'
Katachi::Shapes['ref:bar'] = 'bar'

# ^ Effectively this is placing a translation layer between the shape library and the user code