Add a list.count function

[Quantumplation]

Description

Add a list.count function that counts the number of items that match a predicate.

Motivation

You can achieve the same thing with foldl, but it's far more ergonomic to do something like

required <= list.count(scripts, fn(s) { satisfied(s, signatories, validRange) })

vs

required <= list.foldl(scripts, fn(i,t) { if satisfied(s, signatories, validRange) { t + 1 } else { t } }, 0}

Are you willing to submit a PR?

Yes, one coming in just a few minutes :)

(There's no CONTRIBUTING.md, so I wasn't sure whether to open a ticket first or not, and decided to err on the side of caution)