Hello all,

Here's another patch set to improve support for source properties.
In brief:

* 'read' now sets source properties on non-immediate numbers: bignums,
  floats, fractions, complex.

* add the 'supports-source-properties?' predicate (as well as
  scm_supports_source_properties_p), which cannot be implemented
  efficiently in Scheme.

* relax validation checking of source property getters so that they may
  be applied to _any_ object.  Previously, attempts to get source
  properties of immediate objects would throw an error.

* psyntax now accesses and sets source properties for all supported
  objects.  Previously it assumed that only pairs could support source
  properties.

* add tests to verify that 'read' sets source properties appropriately.

For ease of reading, the first patch shows only non-whitespace changes,
since the bodies of three functions changed indentation level.

I very nearly pushed this, but wanted to make sure there were no
objections to adding 'supports-source-properties?', or to relaxing the
validation of source property getters.

What do you think?  Okay to push?

   Thanks,
     Mark