Expand description
exp and ln for every pure path (spec 047).
The platform’s math library differs between targets (glibc, musl, Apple,
Microsoft and Rust’s own on WebAssembly) in the last bit of some results,
so the pure crates never call f64::exp or f64::ln. These functions
are one pure Rust implementation, the libm crate exact-pinned without
default features, and give the same bits on every target. They are not
correctly rounded, so they stay outside exact computation (spec 002,
3.4.6); a test (tests/platform_math.rs) refuses a direct call.