allow constructor names in substrate

[xermicus]

This changes a few things regarding constructor names for the substrate target:

• Constructors may now have a name, like functions.
• If they have no name, new will be used automatically.
  • It is a convention to call "constructors" new in Rust too
  • new is a reserved keyword in Solidity, so you can't write a function called new anyways (prevents conflicts)
• It works nicely together with the function name mangling.

To make instantiation of ink! (Substrate) contracts possible in the future, I think generating interface files is the better solution than changing our syntax to allow it.

[seanyoung]

Just some minor documentation comments. Looks good!

[LucasSte]

Just one nitpick.