Fix bigdecimal spec

[tompng]

BigDecimal v3.3.1

Fix for new feature BigDecimal(float)

BigDecimal(1.2)
# before: ArgumentError
# after:  0.12e1

Fix assertion of BigMath.log not to expect incorrect random digits to be appended.

BigMath.log(2, 10)
# log(2): 0.693147180559945309417232121458176568075500...
# before: 0.693147180559945309417232112588603776354688e0
# after:  0.6931471806e0

Fix assertion of too-strict precision and assertion that expecting precision to be too loose.

BigDecimal('1.2')**-1
# before: 0.833333333333333333333333333333333333333333333e0
# after:  0.83333333333333333333333333333333e0

BigDecimal(0) + 1/3r
# before: 0.333333333e0
# after:  0.33333333333333333333333333333333e0

Fix divmod, modulo and remainder assertion which was expecting a bug inconsistent with Float as a specification.

[BigDecimal(1).divmod(-BigDecimal::INFINITY), 1.divmod(-Float::INFINITY)]
# before: [[0.0, 0.1e1], [-1, -Infinity]]
# after:  [[-0.1e1, -Infinity], [-1, -Infinity]]

BigDecimal(1).remainder BigDecimal(0)
# before: NaN
# after:  ZeroDivisionError
# Float#remainder and Integer#remainder: ZeroDivisionError

Remove coerce expectation to add/sub/mult which was dropped because of a bug, and never implemented correctly in BigDecimal.

BigDecimal(2).add(0i, 0).class
# before: Calls coerce and returns Complex
# after:  Raise TypeError without calling coerce

Unlike + - *, add sub mult is required to calculate within BigDecimal and to return BigDecimal.
a.coerce(b) #=> [b2, a2] only guarantees that a2.class == b2.class. Numeric's default behavior is to return [Float, Float]. It's not suitable to convert something to BigDecimal.

[eregon]

Could you use a version_is guard instead? (see usages of that for examples)
skip should only be used as last resort if there is no other way, i.e. skip should be avoided as much as possible.
Maybe we should doc that better for skip, how did you find skip BTW?

[eregon]

Thank you for the PR, it looks good in general.

[eregon]

Use version_is instead, and same for other usages of skip