erc20/v0 package

Package erc20 implements MiniToken, a minimal fungible token modeled on the Solidity ERC-20 idea, ported to gno.land (gno 0.9 / sapphire).

State is package-level and persistent. All state-mutating exported functions are crossing functions (they take `cur realm` as the first parameter) per the gno 0.9 interrealm convention, and identify the caller via cur.Previous().Address() after checking cur.IsCurrent().