Apply transforms correctly - #230

`schemars` only applies the transforms from its settings inside
`root_schema_for` and `take_definitions(true)`. `aide` calls neither when
building operations: it uses `subschema_for` and reads schemas back via
`GenContext::resolve_schema`, so every schema `aide` inlines into the
document was emitted untransformed and the generator settings were
silently ignored.

This is not limited to custom settings. `aide` defaults to
`SchemaSettings::draft07`, which ships `ReplaceUnevaluatedProperties`,
`RemoveRefSiblings` and `ReplacePrefixItems`, so a query struct with a
tuple field emitted draft 2020-12 `prefixItems` into a draft 7 document.

Add `GenContext::resolve_schema_transformed` and
`GenContext::apply_transforms`, and use them wherever a resolved or
inline schema is embedded directly: `parameters_from_schema` (which
covers `Path`, `Query`, typed routing and `serde_qs`), the request body
and response media types for `Json` and both `Form` extractors, and the
header schema for `TypedHeader`.

Schema references are deliberately left alone, since the definitions they
point at are already transformed by `take_definitions(true)` when they are
merged into the document.

`TypedPath` needs care in the other direction: it reads its description
out of `root_schema_for`, which has already run the transforms, so reusing
that schema for the parameters would transform them twice. `Transform`
carries no idempotency guarantee, so it now takes the parameters from
`subschema_for` like every other caller of `parameters_from_schema`.

Supersedes #230, which fixed `parameters_from_schema` but whose
`operation_input_json` hunk transformed a clone that was only read for its
description and then discarded, and which left the other five sites
untouched.

Co-authored-by: Diggory Blake <diggsey@googlemail.com>