Checked arithmetics

Was thinking about checked/unchecked math, and I think the easiest would be making the default syntactic sugar reduce to the checked math implementation, and the unchecked version would require the user to explicitly use `Unchecked.add(x, y) -> z`

The `unchecked {}` block syntax in solidity is nice, but not sure yet it is worth the added complexity (tracking mode within sub-blocks). It is rarely used for need, in many cases it is used for hyper-optimized code. And those optimization cases ideally can be solved by a smarter compiler/language.

The issue is a design proposal for checked and unchecked arithmetic syntax in the experimental Solidity compiler. Start by locating the compiler's arithmetic syntax and implementation, then determine how checked arithmetic is currently represented. Done means the default arithmetic behavior and explicit Unchecked.add form are specified and implemented, with the treatment of unchecked blocks resolved.

Written by the indexing model from the issue text.