2021 edition bump + doc improvements; MSRV 1.56 (#80)

- Bumps both `spake2` and `srp` to Rust 2021 edition
- Uses the new `doc = include_str!(...)` attribute to include README.md
  files in the rustdoc
- Improves the README.md files, adding an initial one for `srp`
- clippy fixes for Rust 1.56

1 files changed, 0 insertions, 1 deletions

diff --git a/srp/tests/mod.rs b/srp/tests/srp.rs
index 58bfa0f..bf6c30f 100644
--- a/srp/tests/mod.rs
+++ b/srp/tests/srp.rs
@@ -1,4 +1,3 @@
-use rand;
 use rand::RngCore;
 use sha2::Sha256;