Built-in SDD hooks

[dekarrin]

FISHI should provide built-in implementations for commonly-used SDD hooks. These would be of the following type:

• Produce a constant value with no arguments, somefin like %const "the const" for the hook, where the value of const is any valid value to be interpreted in Go.
• The identity value. Easily implemented as simple as %ident attr-ref, but would be ideal to simply allow omission of the actual hook function and if it is instead an AttrRef, assume it is identity (such as {^}.value = {1}.$text.