Replace Swash?

[dhardy]

Swash implements a variety of text- and font-processing functionality, of which we use a subset (mainly text analysis and font rastering). Development appears to have stalled (see dfrg/swash#107).

[dhardy]

Remaining work: make the swash dependency optional (a breaking change); it is now used only by the font library (to support down-stream usage for rastering).