Adds methods to gracelib, usees inheritance.

Adds methods to object, string and math.   Built-in objects in
gracelib.js now benefit from Grace inheritance, using the superobj
link.  mirrors.reflect now looks at the superobj link.

Some shared method properties were moved out of prototypes,
where they were being extended when methods were added to
instances.

Perhaps because of these changes, type tests (with match) now
seem to work!

Tests for math methods (t151) and for string methods (t160) added.
Note that the case-change methods are now called “asUpper”,
 “asLower”, and “capitalize”.

Names of the basic iterator methods changed to ‘iterator’ and
‘hasNext’  (rather than ‘iter’ and ‘havemore’).