Add string-ascii and string-utf8

[friedger]

This PR

• adds string-ascii and string-utf8 to the list of atomic types and to the list of possible types for map key and value types
• addresses #3

[friedger]

Strings are implemented on the stacks chain through https://github.com/blockstack/stacks-blockchain/pull/1779

What is the specification for type admission between buff, string-ascii and string-utf8 ?