Switch to using 'ln' for the natural logarithm and 'log' for arbitrary base logarithms
Implement exp_m1 and ln_1p as methods for Float

Both expm1 and ln1p have been renamed to exp_m1 and ln_1p in order to be consistent with the underscore usage elsewhere.

The exp_m1 method is used for increased accuracy when doing floating point calculations, so this has been moved from the more general 'Exponential' trait into 'Float'.
Fix order of methods
Add abs_sub method to Signed trait
Add is_normal and classify methods to Float trait
auto merge of #6301 : bjz/rust/numeric-traits, r=pcwalton

This is part of the redesign of the numeric traits tracked in issue #4819.

Renamed:

- `Exponential::expm1` -> `Float::exp_m1` - for consistency with underscore usage elsewhere
- `Exponential::log` -> `Exponential::ln` - a less ambiguous name for the natural logarithm
- `{float, f64, f32}::logarithm` -> `Exponential::log` - for arbitrary base logarithms
- `Real::log_2` -> `Real::ln_2`  - for consistency with `ln`
- `Real::log_10` -> `Real::ln_10` - for consistency with `ln`

Added:

- `Signed::abs_sub` - wraps libm's `fdim` function
- `Float::is_normal` - returns `true` if the number is neither zero, infinite, subnormal or NaN
- `Float::classify` - returns the floating point category of the number
- `Float::ln_1p` - returns the natural logarithm of the number plus one