Define and implement a common style for function-like macros

[killercup]

Diesel currently has a bunch of macros that are called like functions, e.g. joinable!, numeric_expr!, operator_allowed!, and sql_function!. In contrast to regular Rust functions, macros can consume arbitrary tokens, so guessing the correct way to call a macros is not always easy.

To make this a bit more consistent in Diesel's code base and user projects, we should settle on some general guidelines. The general idea is to be consistent and not surprise users with weird syntax.

• (How) should we allow trailing commas in argument lists?
• How are mappings (between types, tables, Rust and SQL, etc.) written?