feat: add constructorCall to sol!

[leruaa]

Motivation

I needed to encode constructor args to create a contract creation tx, and saw #482.

Solution

In sol-types

• Adding SolConstructor trait
• Handle constructor in function::expand() (creating a constructorCall struct that impl SolConstructor)

In json-abi

• impl ToSol for Constructor
• Adding AbiFunctionKw::Constructor variant
• Update fixtures in "tests/abi"

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[DaniPopes]

LGTM, thanks!