Use doubles instead of ints in R for igraph_integer_t

[szhorvat]

Values which have type INTEGER in functions.yaml should be handled as type numeric, and not type integer, in R. This is so that we can support values greater than $2^{31}-1$.

This change needs to be done in:

• Stimulus code generator
  • On the R side, use as.numeric instead of as.integer
  • On the C side, retrieve value as REAL (i.e. double type) instead of INTEGER, then converted to igraph_integer_t
  • When converting from a double to igraph_integer_t, we need to check that the following holds and throw an error otherwise:
    • The value fits in an igraph_integer_t, i.e. IGRAPH_INTEGER_MIN <= value && value <= IGRAPH_INTEGER_MAX.
    • The integer value was exactly representable in a double to start with. Check that the value is no greater than IGRAPH_MAX_EXACT_REAL. This constant is currently internal, found in cigraph/src/math/safe_intop.h.
• Do the same as above for the hand-written code in rinterface_extra.c. Pay attention that for some functions, only the R code, or only the C code is auto-generated, but not both. Thus the update needs to be done simultaneously for the auto-generated code and the hand-written one.

I have already updated most of the hand-written code where possible, but I did not add overflow checks for the double -> igraph_integer_t conversion. This should be done as well, as the last step.

[szhorvat]

@krlmlr It seems to me that the parts which are essential for a 2.0 release are done. I'm removing this from the upgrade milestone. Re-add it if you disagree.