Make PDL be a C library with a Perl interface so it can be used from other C (or dynamic language) code

Tasks:

• segregate the Perl-using C code (mostly pdlcore.c) completely away from the "PDL API" code, separating PDL_CORE_LIST into PDL_PERL_LIST and PDL_API_LIST - for compatibility reasons this will need to remain called "pdlcore.h" so may as well still call the .c the same, and probably merge pdlperl.h back in; the C API stuff would need to move to pdl.h
• make macros that handle either allocating or using the default arrays such as def_dims
• separate out a PDL_Value union type for the PDL_Anyval.value
• use that to add a pdl.value entry to be used for ndarrays whose data segment is less than that number of bytes (which for small types could be several elements) obviously including scalar/single-value, using the alloc/default macro
• switch PDL to just using realloc rather than pdl_grow/pdl_makescratchhash
• have a pdl_impl_vtable * pdl.impl pointer, and void * pdl.impl_data that would replace the current datasv, sv, hdrsv (probably with a struct for the Perl implementation to use), implementing a Perl version
• segregate remaining croak (etc)-using code into that vtable (especially pp_indterm)
• that vtable might also want memory-management entries (malloc, realloc, free) unless the "switch to realloc" item renders it moot
• eliminate the whole pdl.hdrsv from the C code, make the Perl object routinely be a hashref with a PDL and a hdr element so any copying could be done at Perl level
• detection of POSIX threads could be de-Perl-ised

This is somewhat connected to the #349 ideas on making a broadcastloop vtable, but only somewhat.

Start by reading pdlcore.c, pdlcore.h, pdlperl.h, and pdl.h to map the existing Perl-dependent code and API declarations; pp_indterm and the related #349 broadcastloop-vtable ideas are also called out. Done means the unchecked architectural tasks are implemented, including a separated C API, a Perl implementation behind the vtable, and removal of the listed Perl-specific dependencies from the C API.