Compile string builtins

[vaivaswatha]

substr, strlen, to_string are pending at the time of writing this.

[vaivaswatha]

• substr: https://github.com/Zilliqa/scilla-compiler/commit/0a3cd5b01688fa65a4431bb9fe644bec19bb9555
• strlen : https://github.com/Zilliqa/scilla-compiler/commit/f89ae524e3d295a10f882226caa48c59e5efe2ed
• to_string: https://github.com/Zilliqa/scilla-compiler/commit/fd34b87e83b0a75cd6c1c7e5f7d96d6bc0ae1b3f