forme 0.6.0

Bounded shaping through a clone the caller already owns, at the allocation
cost of unbounded shaping. Closes #882, filed against 0.5.0.

Face.ShapeGlyphsBounded is ShapeGlyphsContext on the receiver, for a
caller that keeps its own clones, one per goroutine:

• It makes no clone. The glyphs it shapes are recorded on the receiver, as
  ShapeGlyphs records them, for the caller to merge. A run that fails may
  have recorded some before it stopped.
• Its budget is kept on the face and reset each call, so the bounds
  allocate nothing per run. Clone does not share it.
• The limits and failures are those of ShapeGlyphsContext, which is now
  this on a fresh clone.
• A face already under a budget, such as the one WithShapingLimits hands
  out, refuses it.

The context is now asked every 64 work charges, not at each one, and still
at every boundary between phases. Each ask was an interface call, and a
lock for a cancellable context.

On 14-character NotoSans labels with a cancellable context (medians):

unbounded 8.5 µs 2,784 B 19 allocs
ShapeGlyphsContext, 0.5.0 11.2 µs 4,287 B 26 allocs
ShapeGlyphsContext, now 10.0 µs 4,286 B 26 allocs
ShapeGlyphsBounded on a clone 9.3 µs 2,784 B 19 allocs

The whole suite with every corpus, and go test -race ./..., passed on CI
for the pull request in this release.

Pre-1.0. This adds exported API (ShapeGlyphsBounded) and removes none.