Support serialization of uniform vector literals

Support serialization of uniform vector literals

* libguile/uniform.h:
* libguile/uniform.c (scm_uniform_vector_element_type_code): New
  interface, returns a type code as an integer.

* module/system/vm/assembler.scm (<uniform-vector-backing-store>)
  (simple-vector?, uniform-array?, statically-allocatable?)
  (intern-constant, link-data, link-constants): Support uniform arrays,
  and punt on vectors aren't contiguous from 0.  Support for general
  arrays will come later.

* test-suite/tests/rtl.test ("load-constant"): Add tests.