…warning as an error

As noted by @Benlitz, Cargo metadata supports directly specifying the
target name instead of using the `#[cfg]` syntax. We can support this by
manually creating the `syn::NestedMeta` node from the target name seen
as a string literal.

By checking the recursion depth in the crate we are currently parsing,
we can decide on the right module directory to find submodules in. The
added `mod_2018` test fails without the changes to `parser.rs`.

If the current module is defined in a file called `mod.rs`, it is a 2015
edition module definition file, thus its module directory is the
directory containing the `mod.rs` file. Otherwise, it is a 2018 edition
or single-file module.

By checking the recursion depth in the crate we are currently parsing,
we can decide on the right module directory to find submodules in. The
added `mod_2018` test fails without the changes to `parser.rs`.

If the current module is defined in a file called `mod.rs`, it is a 2015
edition module definition file, thus its module directory is the
directory containing the `mod.rs` file. Otherwise, it is a 2018 edition
or single-file module.

All case conversion cases seem to be covered by this crate so this
greatly simplifies `rename.rs`.

Let the user add raw C code in the generated header after the include
blocks.

It matches what bindgen provides with the raw_lines option.

…d enum methods.