Single-character tate-chu-yoko (unrotated letters and numbers in vertical text)

Following up from #3506 and https://github.com/mapbox/mapbox-gl-js/pull/3438#issuecomment-259611065, when a label is vertically oriented with a vertical writing mode, any one- or two-digit numbers (and one- or two-letter words) would ideally be left upright, their digits placed side-by-side. The same should happen for one- or two-letter runs of Latin text. This effect is called tate-chu-yoko in Japanese:

Currently, we replace a punctuation character with its fullwidth analogue if both adjacent characters are compatible with vertical writing. This heuristic is conservative enough that we could implement single-character tate-chu-yoko simply by mapping all the ASCII numbers and letters to their fullwidth analogues in lookup (or leftAndRightBinding once #3587 is implemented), or by adding 0xfee0 to the character code of any ASCII number or letter.

/cc @lucaswoj @nickidlugash @friedbunny

Start by tracing the vertical-writing punctuation replacement in lookup; if #3587 has landed, compare the related leftAndRightBinding path. Verify the handling of one- and two-character ASCII number and Latin-letter runs in vertical labels, and confirm the existing vertical-writing behavior remains intact.