Derivation functions (hard/soft for pair, soft for public)

[jacogr]

Tested via cargo test by swapping the #[wasm_bindgen_test] to #[test] for hard_derives_pair, soft_derives_pair & soft_derives_public, there are no WASM-specific tests included here.

This basically is the code from https://github.com/polkadot-js/wasm/blob/master/packages/wasm-schnorrkel/src/lib.rs#L64-L104 adjusted fitting in with the conventions used here

Addresses #9

With the above macro swap, this is my outputs -

 $ cargo test
   Compiling schnorrkel-js v0.2.0 (/Users/jacogreeff/Projects/paritytech/schnorrkel-js)
    Finished dev [unoptimized + debuginfo] target(s) in 4.52s
     Running target/debug/deps/schnorrkel_js-13f16448e9949324

running 3 tests
test tests::hard_derives_pair ... ok
test tests::soft_derives_pair ... ok
test tests::soft_derives_public ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

(As mentioned, no JS WASM tests here)

[kianenigma]

You could also test this with either of wasm-pack test --node or --chrome (second one needs a line of change).

 [1/5] Checking `rustc` version...
  [2/5] Adding WASM target...
  info: component 'rust-std' for target 'wasm32-unknown-unknown' is up to date
  [3/5] Compiling tests to WASM...
     Compiling schnorrkel-js v0.2.0 (/home/kianenigma/Desktop/Parity/schnorrkel-js)
      Finished dev [unoptimized + debuginfo] target(s) in 0.38s
  [4/5] Installing wasm-bindgen...
  [5/5] Running tests in node...
      Finished dev [unoptimized + debuginfo] target(s) in 0.04s
       Running target/wasm32-unknown-unknown/debug/deps/schnorrkel_js-e4072c135dde42d9.wasm
running 7 tests                                     
  
  test schnorrkel_js::tests::soft_derives_public ... ok
  test schnorrkel_js::tests::soft_derives_pair ... ok
  test schnorrkel_js::tests::hard_derives_pair ... ok
  test schnorrkel_js::tests::can_verify_message ... ok
  test schnorrkel_js::tests::can_sign_message ... ok
  test schnorrkel_js::tests::can_create_secret ... ok
  test schnorrkel_js::tests::can_create_keypair ... ok
  
  test result: ok. 7 passed; 0 failed; 0 ignored

what was the reason?

[jacogr]

Since I have specific require-as-compiled-and-execute-JS node and jest tests (the second being problematic for me in the past for various reasons), I never use the WASM-pack variants manually (Same on the Travis CI that builds and publishes)

[jacogr]

(The same JS tests actually get used against asm.js versions as well - that is a whole other battle to get going smoothly)