CLI: Automatic WASM discovery and optimization

[koxu1996]

This PR introduces automatic session code discovery (and optimization), based on project structure - fix for #163. See how it works now:

cd ./kairos-session-code && cargo build --release && cd ..
cargo build -p kairos-cli

No need to set PATH_TO_SESSION_BINARIES or run wasm-opt 😌.

In case user didn't compiled session code, he will be instructed to do it:

error: failed to run custom build command for `kairos-cli v0.1.0 (/home/andrew/Projects/casper/kairos/kairos-cli)`

Caused by:
  process didn't exit successfully: `/home/andrew/Projects/casper/kairos/target/debug/build/kairos-cli-cdc69376860e1954/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at kairos-cli/build.rs:32:9:
  WASM directory does not exist: /home/andrew/Projects/casper/kairos/kairos-cli/../kairos-session-code/target/wasm32-unknown-unknown/release/. Please build session code at `./kairos-session-code` with `cargo build --release`; or set `PATH_TO_SESSION_BINARIES` env variable.

NOTE: PATH_TO_SESSION_BINARIES still can be set.

[Avi-D-coder]

we could build the session code for the user instead of telling them how too.

if the env is not set print the rerun if changed directive on the session code dir and then run the cargo build. make sure to use https://doc.rust-lang.org/cargo/reference/build-scripts.html#rerun-if-env-changed on the env too

[koxu1996]

@Avi-D-coder Done in d5fd874.