Investigate feasibility of getting smart contracts on stable about casper-node

[mpapierski]

It is feasible as it turns out that whatever we were using nightly it’s already in stable. It was a matter of adding feature flags to the contract API to hide #![feature(...)] etc.

The issues though:

Using stable rust by default with #![no_std]as we do currently doesn’t work as alloc_error_handler is required - but alloc_error_handler is a nightly feature. So if you want to write a smart contract with no_std you need a nightly compiler.
Building with stdlib enabled by default increases gas cost and all the test contracts grow in size (approx ~200KB extra for all contracts combined). The overhead is likely from the panic behavior from rust’s runtime compiled into wasm. I.e. overflow checks, stdlib’s panic handler with stack unwinding, etc. We don’t need all that under wasm.
I was able to use stable rust with std enabled on all contracts by disabling most if not all rust’s runtime checks with an extra cargo profile. This is what smart contract devs should use as well and we should update our reference contracts to do this:

[profile.release-smart-contracts]
inherits = "release"
codegen-units = 1
opt-level = "s"
lto = true
debug = false
panic = "abort"
overflow-checks = true

With the above settings, I was able to have stable rust, stdlib enabled, with the same size and same gas cost as the current dev with nightly:

# stable, with default release build flags (initial attempt; bad)
$ wc -c target/wasm32-unknown-unknown/release/*.wasm | grep total
 6099831 total

# stable, stdlib with extra compiler flags
$ wc -c target/wasm32-unknown-unknown/release/*.wasm | grep total
 4800659 total

# dev, nightly, the baseline
$ wc -c target/wasm32-unknown-unknown/release/*.wasm | grep total
 4800659 total

What’s left are the feature flags we want to add to the contract API crate. In my experiment, I added the std feature always enabled, nightly flag, and the wee_alloc flag in my experiment. By default, std is on and it also enables wee_alloc. For convenience, I have a no_std flag that enables a nightly flag (as raised above it’s for #![feature(...) and alloc_error_handler).