[rustdoc-json] Make `header` a vec of modifiers, and FunctionPointer consistent
Bumps version number and adds tests, this is a breaking change. I can split this into two (`is_unsafe` -> `header` and `header: Vec<Modifiers>`) if desired.
Rationale: Modifiers are individual notes on a function, it makes more sense for them to be a list of an independent enum over a String which is inconsistently exposing the HIR representation (prefix_str vs custom literals).
Function pointers currently only support `unsafe`, but there has been talk on and off about allowing them to also support `const`, and this makes handling their modifiers consistent with handling those of a function, allowing better shared code.
`@rustbot` modify labels: +A-rustdoc-json +T-rustdoc
CC: `@HeroicKatora`
r? `@jyn514`
