chore: add foo20, a grc20 example smart contract

[moul]

The goal of this PR is to create an example of the ERC20 smart contract in Gno.

It's unclear yet if it will be useful or just an exercise/demo.

• port an existing ERC20
  interface (https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/IERC20.sol)
• implement GRC20
• unit tests
• publish it, test it with friends (locally)

Some ideas that come to mind while working on this:

• should we have some GRCxxx interfaces defined in gno.land/p/grcxxx (just the
  interface)?
• should this ERC20 demo be a complement of r/tokens, should it be
  included (r/tokens can be a register)?

Bonus:

• add a gno.land/p/ufmt with the start of a micro fmt package for format strings

Related with #139
Related with https://gno.land/r/boards:bounties/2
Related with https://gno.land/r/boards:bounties/3

[moul]

Contract published here: https://gno.land/r/moul_foo20_134150

First transfer here: https://gno.land/r/moul_foo20_134150:balance/g1qpymzwx4l4cy6cerdyajp9ksvjsf20rk5y9rtt

[jaekwon]

this is dope!