Remove singleton behaviour from KatSpec and instead provide a KATSPEC member

This should make Finesse more robust to dangling references when used in site-specific packages that register their own instructions in the language spec - see #517 (closed) for more details. This hopefully doesn't break anything in finesse-virgo but will probably break finesse-ligo. The KatSpec() instance simply needs replaced by KATSPEC imported from finesse.script.spec.