ACP: add `scalbn` and `ilogb` float methods

Proposal

Problem statement

The implementation of complex division needs the scalbn and ilogb functions. We'll likely add these as intrinsics, and I wonder whether we want them as methods too.

Motivating examples or use cases

Complex division pre-processes the arguments like so:

https://github.com/rust-lang/compiler-builtins/blob/082387e5bd9fab846b307686ea7a9a23a74d9f94/compiler-builtins/src/float/complex/div.rs#L14-L20

let max = fmax(c.abs(), d.abs());
let mut ilogbw = 0;
if max.is_finite() && max != F::ZERO {
    ilogbw = ilogb(max);
    c = scalbn(c, -ilogbw);
    d = scalbn(d, -ilogbw);
}

The goal is to make overflow/underflow less likely to occur in subsequent operations.

Solution sketch

impl f* {
    /// Multiplies a floating-point value by 2 raised to power `exp`.
    /// A `scalbn` is always exact, and much cheaper than `self * 2.0powi(n)`.
    const fn scalbn(self, n: i32) { /* ... */ }

    /// Extracts the value of the unbiased exponent from the floating-point 
    /// argument num, and returns it as a signed integer value.
    /// 
    /// This is a cheap way of determining the (order of) magnitude of a float.
    const fn ilogb(self) -> i32 { /* ... */ }
}

For code size reasons we can usually use libcalls for the implementation. compiler-builtins already has these implementations, and Miri/const eval (via rustc_apfloat) already has support.

Alternatives

We can keep these functions just as intrinsics and not expose them.

Apparently it's been suggested before to make the << and >> operators on floats call scalbn. On integers shifts correspond to multiplication and division by powers of 2. I'm not a fan of using infix operators for non-obvious operations but it's an idea.

The names are not the best but they are standard.

Having frexp and ldexp might be useful too.

Links and related work

Tangentially related

• IRLO: Add conversions of floating point to / from exponent+mantissa

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start with compiler-builtins/src/float/complex/div.rs and the linked complex-division code to understand the requested scalbn and ilogb use. Review the existing compiler-builtins implementations and rustc_apfloat support, then follow the ACP alternatives about intrinsics versus methods. Done means the libs-api team has resolved the API and implementation direction for these functions.