Principle: The signature is the contract

[josh11b]

Introduces a new principle called "the signature is the contract," where Carbon requires the information needed to use an entity is present in the entity's declaration (its "signature"). The signature is authoritative, and is not modified by the definition body.

[geoffromer]

Moving a Discord discussion here: I find this name quite opaque and non-mnemonic, to the extent that when revisiting the topic after only a few days, I had to skim this PR again to remind myself what the principle is actually about. I'd suggest instead something like "the signature is self-contained" or "the definition doesn't affect the signature".

To me, "the contract" refers to things like preconditions and postconditions (more broadly, everything that you need to reason about whether the calling code is correct), whereas "the signature" refers to the language-semantic content of a function declaration (i.e. everything that you need to reason about whether the calling code is well-typed). So "the signature is the contract" sounds to me like it's saying that Carbon will have statically-checked contracts as a language feature, and even suggests that we're trying to fully embed program correctness into the typesystem.