Proposition of changes into Casper Types to support a Rust SDK for 1.6

[gRoussac]

This PR intends to pull changes made for the Rust SDK. It is adding a std-fs-io default feature and affects several methods or expose them accordingly to be able to compile the client with

cargo build --lib --no-default-features --target wasm32-unknown-unknown

This PR has a strong dependency on PR against node 1.6 casper-network/casper-node#4348

Mainly adds some #[cfg(feature = "std-fs-io")] where it is possible or not to use io/fs, or prevent sending to std with json_pretty_print and returning deploys
Changes the way secret key is loaded in with_payment_and_session()
Publicizes some methods outside the crate
Adds session_bytes to session_executable_deploy_item() and adds check_no_conflicting_arg_types() (might require additional unit tests)
Adds with_bytes() to SessionStrParams
Adds some Clone attributes to some structs
Removes rt-multi-thread, The default runtime flavor is 'multi_thread', but the 'rt-multi-thread' feature is disabled. switching (flavor = "current_thread") to tokio main. Please check this change is legit.

For simplicity for now keeping new get_maybe_secret_key() as one function and not two as per previous PR for 2.0

casper-ecosystem/rustSDK#4
casper-ecosystem/rustSDK#5
casper-ecosystem/rustSDK#13

[zacshowa]

I think, for the sake of expediency in getting this out for 1.6, these changes are fine. However, I want to push for scheduling work for 2.0 to make this an additive feature.

[Fraser999]

We need to resolve the issue mentioned in my comment.

[Fraser999]

LGTM now.