This commit adds two new methods to the `Math` module:

* `Math.log1p(x)`: Computes `Math.log(x + 1)`
* `Math.expm1(x)`: Computes `Math.exp(x) - 1`

These methods are often more accurate than the straightforward
computation, especially when `x` is close to zero.
The corresponding functions, `log1p` and `expm1`, are defined in the C99
standard math library.

[Feature #21527]

`errno` is no longer used after 5073155a178a9f478950afef4f148e44fd14b5d6

Co-Authored-By: Nobuyoshi Nakada <nobu@ruby-lang.org>

As long as "floating point numbers" cannot accurately represent an
irrational number, the result of the natural logarithm cannot be
accurate.  Logarithms with an integer base may have the possibility to
represent more accurately.

[Misc #18891]