string/trim functions about janet

[bakpakin]

We could add this, it's use is common enough to be warranted as part of the core.

For trim implemented with pegs:

(def- trimmer (peg/compile
  '{:space " "
    :no-space (if-not :space 1)
    :main (* (any :space) '(any (* (any :space) (some :no-space))))}))

(defn trim [str]
  (if-let [matches (peg/match trimmer str)]
    (matches 0)))

Implementing triml and trimr via pegs would be a good exercise for the peg docs.

[quan-nh]

I think space could be any white-space characters, so for peg

:space (+ " " "\t" "\n" "\v" "\f" "\r")

[bakpakin]

Added in 232ea22.