Make `log` work on bignums

[jpellegrini]

Currently, STklos always answers +inf.0 for (log x) if x is a bignum.

Here's a list of what other implementations do: https://github.com/schemedoc/surveys/blob/master/surveys/log-requires-floating-point.md

We can use some logarithm properties and the GMP function mpz_get_d_2exp to get logs of bignums.

• If a bignum fits a double, it will use the usual fast method, without calling the GMP

• Numerators and denominators of rationals are treated separately, so if one needs the GMP, the other may still be computed quickly (and later only a double subtraction is needed)

• Log will still not work if the result of the operation does not fit a double (in which case it will answer +inf.0).

MINI-GMP

The function mpz_get_d_2exp is only available from the full GMP. The mini-gmp does not offer it. So, if the user compiled STklos with the mini-GMP, the behavior will be as it was before this patch (if the number is too large for a double, +inf.0 will be returned).

TESTS

Tests are included.

The fact that we treat mini-GMP and full-GMP differently means that conditional compilation is necessary. But then, in order to write proper tests, it is also necessary to come up with a predicate that can be used to tell if a test makes sense or not: %stklos-has-gmp?.

OTHER RELEVANT FUNCTIONS

• It doesn't seem to make sense to extend trigonometric functions to bignums. While it is reasonable to take the logarithm of a huge number, that is not the same for sin, cos, tan. The same argument applies to asin and acos.

• ~Atan: since (atan n) will converge to $\pi/2$ when n goes to +inf.0, there is not much benefit in allow bignums as arguments.~ --actually, STkos already answers an inexact $\pi/2$ for all bignums! :)

• Hyperbolic trigs: it would be nice to enhance them, but most implementations do the same as we do.

• exp (not expt) could certainly be enhanced, but it's harder than log (we don't have a GMP function that does almost all the work)

• sqrt of bignums currently only work for perfect squares (as in several other implementations) - but this can be improved... This is a good candidate for a next PR (Anyway, most implementations do only sqrt of perfect squares for bignums) ----- update I have a patch ready. Will make the PR later

[jpellegrini]

@egallesio I have also made something similar for SQRT. If you want I may include it as another commit in this PR. (A survey of systems that compute square roots of some bignums is also in the same link above, https://github.com/schemedoc/surveys/blob/master/surveys/log-requires-floating-point.md)