Make trans vtables more visible to Perl, enable kernels to be passed as callbacks

[mohawk2]

In order to better support both #349 and PDL libraries that want to pass callbacks (PDL::LinearAlgebra, PDL::GSL::MROOT, among others), it would be good to make transformations' vtables more of a first-class entity. Things I can think of needed:

• have a package array variable listing all the xforms
• have a Perl class method that can instantiate a vtable
• store the various types of Code in the vtable
• make those available as Perl methods, maybe including an as_hashref method
• break out the per-gentype code snippets into separate kernel functions, put them in a vtable array entry
• allow a vtable object to be passed as a callback for e.g. PDL::GSL::MROOT - this would mean not needing to enter Perl space for each callback-call

This also links to #505.

[mohawk2]

As noted on https://sourceforge.net/p/pdl/mailman/message/59118263/ - this would help with both loop fusion (if a single-step version of readdata/writebackdata is also generated), and implementing "einops". Both the latter would be "meta-functions".

[mohawk2]

To bring over the thought from #505 to enable closing that:

Another thing that would help generally (including for callbacks) is being able to stack-allocate a struct pdl and then call an inline function (which would live in pdl.h) to initialise it; this could then also be used in pdl_pdlnew itself, so there'd be only one "source of truth" for initialisation. There should also be a slightly more powerful version that also initialises dims and datatype, and a further one that also initialises with data (usable by this).