Remove stdlib.sol

[fare]

The functions in stdlib.sol are ultimately not needed:

• We never need to convert things to bytes outside of computing a digest, and this doesn't require any function in stdlib.sol.
• Conversion to bytes and digests is per type, anyway, and just uses abi.encode or in some cases abi.encodePacked, or a similar thing that we'd write if generated EVM code.
• We never actually need to decode from bytes during normal operations, though we need the basic (compiler-generated) runtime to do it for us for initial contract invocation, and we might keep that first-class for reflection.
• We can save on that file, and on having the compiler having to find it at its runtime.