Allow user to rename fields and types

[djc]

serde has a number of ways to allow users to rename fields and types. On struct types, we want to enable:

• #[xml(rename = "foo")] struct Foo {}, which would replace the use of Foo with foo
• #[xml(rename_all = "PascalCase")] struct Foo {}, which would rename all fields in struct according to the named convention

For fields, we just want to support #[xml(rename = "foo")]. Remember that these can compose, so you might encounter #[xml(rename = "foo", namespace("URI"))].

We'll want to use the heck crate for these conversions. Except for Title Case, I think we want to support all four of heck's support variants.

[djc]

If we do enum-as-scaler support before this, should implement rename and rename_all for enums, too.