message: Update README.md alternatives section for apint
  The apint crate is stable as of Rust version 1.26.
diff: @@ -53,7 +53,7 @@ table offers a brief comparison to a few alternatives.
  | [`ramp`] | Apache-2.0 | nightly | rust and inline assembly |
  | [`rug`] | LGPL-3.0+ | 1.18 | bundles [GMP] via [`gmp-mpfr-sys`] |
  | [`rust-gmp`] | MIT | stable? | links to [GMP] |
  -| [`apint`] | MIT/Apache-2.0 | nightly | pure rust (unfinished) |
  +| [`apint`] | MIT/Apache-2.0 | 1.26 | pure rust (unfinished) |
  [GMP]: https://gmplib.org/
  [`gmp-mpfr-sys`]: https://crates.io/crates/gmp-mpfr-sys
  