Improve strides handling

[Barry Schwartz]

Improve the handling of strides to eliminate the possibility of writing Guile code that does strange things in the C code.

Essentially this is a problem with the wild mix of integer ranges under multiple names that is available in C. One (unpleasant) solution is to use pointer arithmetic more.

The priority is low because only code producing wildly incorrect values could trigger such odd behavior in the first place. We are talking about stuff such as polynomials of such enormous degree that the program likely would have crashed already merely trying to store them, and which would be useless once stored.

[Barry Schwartz]

Won’t fix.

[Barry Schwartz]

Notably we now use size_t and ssize_t integers where we used to use unsigned int and int, so now indexing is ‘safe’ on a scale comparable to the size of virtual memory.