srp: rebuild library (#79)

Complete rewrite of the SRP library.

Includes many improvements over the old library:

- Improved file and code organization
- Access to individual SRP computations 
- Consistent sever and client API
- Simpler API
- Improved documentation with tests in documentation
- New tests for compatibility with the RFC
- Bumps dependencies
- Timing safe verification comparisons
- Modernized error handling

srp/Cargo.toml

@@ -9,19 +9,19 @@ repository = "https://github.com/RustCrypto/PAKEs"
 keywords = ["crypto", "pake", "authentication"]
 categories = ["cryptography", "authentication"]
 readme = "README.md"
-edition = "2018"
+edition = "2021"
 rust-version = "1.56"
 
 [dependencies]
 num-bigint = "0.4"
 generic-array = "0.14"
-digest = "0.9"
+digest = "0.10"
 lazy_static = "1.2"
+subtle = "2.4"
 
 [dev-dependencies]
 rand = "0.6"
-sha2 = "0.9"
-sha-1 = "0.9"
-
-[badges]
-travis-ci = { repository = "RustCrypto/PAKEs" }
+sha2 = "0.10"
+sha-1 = "0.10"
+num-traits = "0.2"
+hex-literal = "0.3"