refactor: build chain extension method

[chungquantin]

Link to discussion: #71 (comment)

Contract size: Original wasm size: 31.2K, Optimized: 7.5K

pub(crate) fn build_extension_method(
	version: u8,
	function: u8,
	module: u8,
	dispatchable: u8,
) -> ChainExtensionMethod<(), (), (), false> {
	ChainExtensionMethod::build(u32::from_le_bytes([version, function, module, dispatchable]))
}

[evilrobot-01]

An alternative implementation which keeps things simpler imo, taking advantage of scopes: ee4d499

• reuses existing lib.rs rather than introducing a new module for a single function. build_extension_method will probably be used in almost every submodule, so easier if its globally available rather than within a separate utils module.
• adds a build_extension_method at the v0 module root, so we dont have to repeat V0 everywhere within the module
• adds simple build_dispatch and build_read_state functions as well, to avoid having to repeat the DISPATCH and READ_STATE constants throughout all the implemented functions. We could drop the build_* prefix on these functions too.
• no pub(crate) modifiers required

[Daanvdplas]

Could you also add the contract size in the description of the PR after the changes made (with --release)

[evilrobot-01]

Looking good, think there are a few final possible improvements and then should be good to merge.

[evilrobot-01]

Looking nice and clean! One final improvement suggested.

[evilrobot-01]

LGTM, thanks for your patience! This will be a very important piece to everything that needs to be built out, so great to have such a tidy implementation.

[Daanvdplas]

I think something went wrong when merging 'daan/api' because the helper functions are now not being used in the fungibles apis.

[chungquantin]

@Daanvdplas Rebased. Should be ok now. Can you check it again?

[Daanvdplas]

Good work @chungquantin!