Add support for Solidity using the Solang compiler

anchor init and anchor new have a new option `-s` (or `--solidity`) which
creates an example using Solidity in the soldity directory of the
anchor workspace.

anchor deploy/build/test work accordingly.

solang is required to be in the path, this can be downloaded from the
release at https://github.com/hyperledger/solang

anchor build/deploy finds all the solidity contracts by using the
solidity parser to find all contract declarations.