Fix no-std support

[dtolnay]

Despite having a "std" feature that can be disabled:

float8/Cargo.toml

Lines 30 to 32 in 273b1f9

	[features]
	default = ["std"]
	std = []

the float8 crate did not work with a no-std target platform because it unconditionally enabled the half crate's default "std" feature:

float8/Cargo.toml

Lines 13 to 14 in 273b1f9

	[dependencies]
	half = "2.4.1"

and also unconditionally used a bunch of APIs from std, which no-std platforms do not have.

float8/src/lib.rs

Lines 83 to 87 in 273b1f9

	use std::{
	cmp::Ordering,
	num::FpCategory,
	ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Rem, RemAssign, Sub, SubAssign},
	};

error[E0463]: can't find crate for `std`
  |
  = note: the `riscv64gc-unknown-none-elf` target may not support the standard library
  = note: `std` is required by `half` because it does not declare `#![no_std]`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

[EricLBuehler]

Thank you!