exp/upbox: add SchemaFromYAML function
exp/upbox: add SchemaFromYAML function

Change-Id: Ie32155cce37e6edf9f4fc358ecbfebef539a4f1e
Reviewed-on: https://upspin-review.googlesource.com/10066
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/exp,upspin/exp