feat: negative to unsigned int conversion

[bsh98]

Motivation

Resolves https://github.com/quilt/etk/issues/79

Approach

Added uintN(), a built-in expression macro to convert negative evaluating expressions to positive values. A new expression type Uint was added to account for the case when the expression needs to be evaluated with context (e.g. user-defined macro). Any feedback is greatly appreciated!

Example

push1 uint8(-1) evaluates to 0x60ff.

Alternatives

It may be a bit cleaner to just add built-in macros for max unsigned integer values (e.g. max_uint256()). I could also see some benefit to implementing bitwise operators (e.g. 1 << 256). However, the current approach also allows for representing negative integers in two's-compliment, which could be of value.

todo

Will update docs if changes are deemed appropriate.