[feature] #2161: generate FFI functions for data_model

[mversic]

Description of the Change

You can find a description here: #1973

This PR introduces an ffi crate which holds a macro ffi_bindgen. This macro when attached to the impl block generates the equivalent ffi out of the methods listed in the impl block. The macro can also be attached to the structure definition where it integrates with getset to make ffi fn equivalents of the getters/setters produced by the getset.

Note:

1. All FFI functions return FfiResult return code. The idea is that the returned code can be used to query the error
2. Considering the previous points, all FFI functions take, as a last argument, a mutable pointer where the return value of the corresponding method is stored (the so called out-pointer pattern)

Limitations:

1. mutable references are not supported for input arguments, except for the self argument, i.e. handle

Issue

Closes #2161

Benefits

Possible Drawbacks

Usage Examples or Tests [optional]

#[derive(Getters)]
#[getset(get = "pub")]
struct Struct {
    elem: u32,
    #[getset(skip)]
    elems: Vec<u32>,
}

#[ffi_bindgen]
impl Struct {
    pub fn new(elem: u32) -> Self {
        Self {elem, elems: vec![]}
    }
    pub fn get_elems(&self) -> impl ExactSizeIterator<&u32> {
        self.elems.iter()
    }
}

fn main() {
    let s = struct_new(42);
    let elems = MaybeUninit::<*const *const u32>::uninit();
    let elems_len = MaybeUninit::<usize>::uninit();
    if FfiResult::Ok != unsafe {struct_elems(&s, elems.as_mut_ptr(), elems_len.as_mut_ptr())} {
        // TODO: Handle error
    }
    let elems_len = unsafe {elems_len.assume_init()};
    let elems = unsafe {elems.assume_init()};
}

Alternate Designs [optional]

List out ffi functions manually. This is tedious and highly error prone because we're operating in the unsafe space

[codecov]

Codecov Report

Merging #2211 (92c9f2f) into iroha2-dev (fe33f0f) will increase coverage by 0.80%.
The diff coverage is 83.28%.

@@              Coverage Diff               @@
##           iroha2-dev    #2211      +/-   ##
==============================================
+ Coverage       63.89%   64.69%   +0.80%     
==============================================
  Files             127      131       +4     
  Lines           23659    24589     +930     
==============================================
+ Hits            15117    15909     +792     
- Misses           8542     8680     +138     

Impacted Files	Coverage Δ
client/src/config.rs	69.35% <0.00%> (ø)
client_cli/src/main.rs	0.29% <0.00%> (-0.01%)	⬇️
core/src/lib.rs	100.00% <ø> (ø)
core/src/smartcontracts/isi/domain.rs	39.13% <ø> (ø)
core/src/smartcontracts/isi/mod.rs	52.86% <ø> (ø)
data_model/src/permissions.rs	58.62% <0.00%> (-0.64%)	⬇️
data_model/src/role.rs	2.81% <0.00%> (-0.09%)	⬇️
data_model/src/trigger/mod.rs	22.82% <ø> (ø)
tools/kagami/src/main.rs	0.52% <0.00%> (+<0.01%)	⬆️
data_model/src/asset.rs	51.46% <46.00%> (-0.92%)	⬇️
... and 23 more

---

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[appetrosyan]

Please squash this PR with care