Add a BigFloat with const generic precision

[tgross35]

A lot of the time, needed precision might be known ahead of runtime. Const generics might be able to make use of something like this:

struct KnownBigFloat<const WORDS: usize = 2> {
    inner: KnownFlavor<WORDS>,
}

#[derive(Debug)]
enum KnownFlavor<const WORDS: usize> {
    Value(KnownBigFloatNumber<WORDS>),
    NaN(Option<Error>),
    Inf(Sign), // signed Inf
}

pub(crate) struct KnownBigFloatNumber<const WORDS: usize> {
    e: Exponent,
    s: Sign,
    m: KnownMantissa<WORDS>,
    inexact: bool,
}

pub(crate) struct KnownMantissa<const WORDS: usize> {
    m: [Word; WORDS],
}

I expect this would have a noticeable performance increase as well - mantissa is smaller for <= 128 bits of precision, allocation-free, no indirection, and probably some nicer optimizations.

A downside is this isn't as elegant as it could be with the unstbale generic_const_exprs - ideally you could specify BITS in the const generic and store m: [Word; BITS / WORD_BIT_SIZE], but that won't be stable for a while.