Ensure building/checking depends on Wasm import functions being correct

[cmichi]

We want to examine the resulting contract Wasm in order to ensure that certain criteria are met.

ToDo

• Ensure that building/checking happens only if the following conditions are fulfilled.
• Ensure that only seal_* functions are imported in the resulting contract Wasm.
• For functions which are not seal_* we want to give helpful messages:
  • Recommend overflow-checks = false when this bug occurs.
  • If generated ink! constructs like this are found.