Feat: Add math funcs Log10, Log2, Log and Exp

[jrmoulton]

Fixes #755
I'm not sure if the Log10 and Log2 functions need to stay but I added them because both c++ and rust have them as a standard.

[CLAassistant]

All committers have signed the CLA.

[ogoffart]

Great change, thanks!
It just missed tests (in math.60 for example) and documentation (in langref.md)

I wonder if we really need log2 and log10 if we can use log(foo, 2) and log(bar, 10). Also I think exp should take a parameter with the exponent.

[ogoffart]

I realize that C++ and rust and JavaScript calls the exp function with an arbitrary base "pow". So we should have this function as well.

[jrmoulton]

exp shouldn't take a power it's just e^x. But I could also add a pow function for a more generic exponential function which I guess might be what the issue was referring to.

I'd lean towards removing log10 and log2 and just using log. By the same token should I remove exp and just implement a pow function?

[ogoffart]

Looks good, thank you.
We'll merge once the CI passes.

[ogoffart]

Ah, it is still missing the documentation in langref.md
And it also deserve an entry to the changelog.

[jrmoulton]

There we go. That should be all of it (hopefully the changelog entry looks fine)

[ogoffart]

Perfect, thanks!