Soroban: Support for Constructors

[tareknaser]

This PR adds support for user-defined constructors in Soroban, specifically for constructors with no arguments. Note that constructors with arguments are not yet supported (see comments for details).
Part of #1672

Other changes include:

• Refactored code in tests/soroban.rs to allow providing constructors when registering contracts on the chain.
• Added tests for constructors in tests/soroban_testcases/constructor.rs.

[tareknaser]

So adding support for constructors with custom arguments wasn't straightforward. I have two approaches in mind that I thought of, which I’ll explain below.

Let me know what you think, or if there’s a better, more straightforward approach I might be missing.

[tareknaser]

We can set cfg.params to the parameters of the constructor FunctionValue's params. This will likely include a mapping that looks something like:

let mut params = vec![];
for param in constructor.get_params() {
	let param = match param.get_type() {
		BasicTypeEnum::IntType(int_type) => {
			let ty = match int_type.get_bit_width() {
				1 => ast::Type::Bool,
				32 => ast::Type::Uint(32),
				64 => ast::Type::Uint(64),
				_ => unreachable!(),
			};
			ast::Parameter::new_default(ty)
		}
		_ => unreachable!(),
	};
	params.push(param);
}
cfg.params = sync::Arc::new(params);

This won’t work because we still need to pass the arguments to build_call() (on line 296), which, of course, we don’t know.

[tareknaser]

Another approach I have in mind is to get the constructor ControlFlowGraph itself and prepend it with a call to storage_initializer, but this won't work either because the contract is immutable.

We could clone it and re-export it, but I’m not sure if that’s the best approach.

[seanyoung]

Looks good to me, really needs @salaheldinsoliman approval for merging (as he looks after the Soroban target)