feat: add struct parsing support for human readable ABI

[mattsse]

This adds support for parsing solidity structs in the humand readable ABI.
A new AbiParser can parse structs, turn them into tuples and substitute their references in events and functions.
Also this improves the error handling with textual feedback about the error that occurred.
Additional tests are tests are added as well.

Motivation

Support for structs in the human readable ABI makes it easier to copy definitions from your solidity contract without further modifications.

Solution

The SolStruct can parse solidity structs, supported features are:

• elementary datatypes (ethabi::ParamType), like int25 _y;
• struct references as String, like MyStruct m; and MostOuter.Outer.Inner m;
• struct arrays, fixed and unbounded: MyStruct[] m; and MyStruct[10] m;
• mappings and nested mappings like mapping(string => string) and mapping(string => mapping(string => string))

A new AbiParser is introduced that handles struct parsing and replacing them as tuples, either within structs or in event and function arguments.

Substitution is based on the following rules:

• a struct becomes a tuple: struct S {int x; int y;} => tuple(int,int)
• a struct array becomes an array: function(S[] _s) => tuple(int,int)[] and function(S[10] _s) => tuple(int,int)[10]
• nested structs become nested tuples: struct X {S s; address a;} => `tuple(tuple(int,int),address)
• mappings are disallowed and lead to errors in the AbiParser

The parse api remaines unchanged but its code is replaced AbiParser::default().parse(s)

[gakonst]

This is great, thank you.