Define failure semantics for `fact` operator

Defining `fact` to match or raise an error prevents matching failures because
of typos or incorrectly named facts. Allowing a second, default argument for
the `facts` operator prevents "special-case" facts -- facts for specific nodes
that aren't usually defined -- from causing rampant error-raising. The result
helps defend against human error while providing an easily-accessible override.
Refuse to bind nodes if any tag raises an error

To prevent accidental binding of nodes to policies due to typos, no node is
permitted to bind to a policy if a tag raises an error during evaluation.
To aid in troubleshooting, the node appends an error message to its log with
details of the failure.
Add num, gte, gt, lte, lt matcher operators

This adds standard numeric greater-than, less-than, and their respective
-or-equal operators to the tag matchers. Since these currently support only
numeric values, a `num` operator is also provided, which attempts to convert
its argument into a numeric equivalent. While the gt, lt, etc. operators are
useful of themselves, `num` is provided as a continuation of the 'explicit
rather than implicit' pattern.

`num` supports binary, octal, decimal, and hexadecimal integers, and floating-
point numbers in decimal or scientific notation. All values can be passed as
strings or as literal numeric values. Any other values will cause an
ArgumentError to be raised.