[Do not merge] Partial work on #137. Only struct PaHostApiTypeId ported for now.

[clicketyclack]

Did an initial run with bindgen + ported over a small section of the resulting ffi.rs. I.e. only the (Pa)HostApiTypeId enum. Then I fixed breakage such that the devices.rs, hosts.rs and sine.rs examples seem to work.

The existing solution defines a bunch of consts in ffi.rs with a wrapping enum in types.rs. Running bindgen creates an enum right in ffi.rs, giving us two enums with basically the same members.

Since bindgen does the enum stuff, I guess we might as well drop the enum in types.rs altogether? Haven't tried doing that to see what breaks yet.

Looking at the generated .rs, naming and types change a bit. See DeviceInfo struct below. So fixing this issue will break a lot of API. Don't know if RustAudio has a policy on that.

$ diff -y ffi-regen.rs src/ffi.rs
(... snip ...)
pub type PaTime = f64;                                        |
pub type PaSampleFormat = ::std::os::raw::c_ulong;            | #[derive(Clone, Copy, Debug)]
#[repr(C)]                                                      #[repr(C)]
#[derive(Copy, Clone)]                                        | pub struct C_PaDeviceInfo {
#[derive(Debug)]                                              |     pub struct_version: i32,
pub struct PaDeviceInfo {                                     |     pub name: *const c_char,
    pub structVersion: ::std::os::raw::c_int,                 |     pub host_api: HostApiIndex,
    pub name: *const ::std::os::raw::c_char,                  |     pub max_input_channels: i32,
    pub hostApi: PaHostApiIndex,                              |     pub max_output_channels: i32,
    pub maxInputChannels: ::std::os::raw::c_int,              |     pub default_low_input_latency: Time,
    pub maxOutputChannels: ::std::os::raw::c_int,             |     pub default_low_output_latency: Time,
    pub defaultLowInputLatency: PaTime,                       |     pub default_high_input_latency: Time,
    pub defaultLowOutputLatency: PaTime,                      |     pub default_high_output_latency: Time,
    pub defaultHighInputLatency: PaTime,                      |     pub default_sample_rate: c_double
    pub defaultHighOutputLatency: PaTime,                     <
    pub defaultSampleRate: f64,                               <
}                                                             <
(... snip ...)