fix: RelaxNG.read_memory is now reliable

[flavorjones]

What problem is this PR intended to solve?

On both JRuby and CRuby impls, replace the native read_memory method
from XML::Schema and XML::RelaxNG with a ruby method that uses
from_document. read_memory was buggy on both platforms, but
especially on the JRuby impl.

This is comparable in performance on CRuby. From a benchmark taken
before this change:

Warming up --------------------------------------
          Schema.new     1.219k i/100ms
Schema.from_document     1.258k i/100ms
  Schema.read_memory     1.118k i/100ms
Calculating -------------------------------------
          Schema.new     12.160k (± 8.3%) i/s -    121.900k in  10.093638s
Schema.from_document     12.216k (± 8.6%) i/s -    122.026k in  10.059696s
  Schema.read_memory     12.790k (±10.7%) i/s -    127.452k in  10.105931s

Comparison:
  Schema.read_memory:    12789.6 i/s
Schema.from_document:    12215.9 i/s - same-ish: difference falls within error
          Schema.new:    12160.1 i/s - same-ish: difference falls within error

IMHO the resulting code is less buggy and easier to maintain, and the
slight (if any) performance hit is worth it.

Also: improve documentation for Schema and RelaxNG classes.

Closes #2113
Closes #2115

Have you included adequate test coverage?

Does this change affect the behavior of either the C or the Java implementations?