math: use libc wrappers for math.log2/1, math.log10/1, math.log1p/1 and math.log_b/1. Make assert math.log10(10) == 1.0 pass.

[spytheman]

The problem was discovered by @Le0Developer, in https://github.com/vlang/adventofcode/pull/71/files#diff-aa6c500b5be6ff2273db2523c71ebe791319dc63602e3857013e33cee163e23cR14 .

_{Huly®: V_0.6-21566}

[spytheman]

assert math.log10(10.0) == 1.0 will pass on this PR by default now, because of the libc wrapper in log.c.v, but will still fail for ./v -exclude @vlib/math/*.c.v vlib/math/log_test.v.

That imho needs more investigation, by people with background in Numerical Analysis.

To me, it seems that the pure V approximation in https://github.com/vlang/v/blob/master/vlib/math/log.v#L122 may need tweaking of some consts, or an algorithmic adjustment.