add missing #[inline] attributes for f32 and f64 in primitive.rs

I testing this change on the following simple program:

```
pub mod primlist_capnp {
    include!(concat!(env!("OUT_DIR"), "/primlist_capnp.rs"));
}

pub fn main() {
    let mut scratch_space = vec![0u8; 16000];

    let mut input: Vec<f64>= vec![0.0;1500];

    let mut cc = 1.1;
    for _ in 0 .. 500000 {
        for inp in &mut input[..] {
            *inp = cc;
            cc += 0.02;
        }
        cc -= input.len() as f64 * 0.02;
        cc += 0.001;
        let allocator = capnp::message::ScratchSpaceHeapAllocator::new(&mut scratch_space);
        let mut message = ::capnp::message::Builder::new(allocator);
        let root : primlist_capnp::foo::Builder = message.init_root();
        let mut list = root.init_float_list(input.len() as u32);

        for (i,n) in input.iter().enumerate() {
            list.set(i as u32,*n);
        }

        for (i,n) in input.iter().enumerate() {
            assert_eq!(*n, list.get(i as u32));
        }
    }
}
```

Adding the #[inline] attribute cut the run time from ~2.5 seconds to about 1.0 seconds.

Author: dwrensha

capnp/src/private/primitive.rs

@@ -65,10 +65,12 @@ primitive_impl!(f64, 8);
 impl Primitive for f32 {
     type Raw = Self;
 
+    #[inline]
     fn get(raw: &Self::Raw) -> Self {
         Self::from_bits(raw.to_bits().to_le())
     }
 
+    #[inline]
     fn set(raw: &mut Self::Raw, value: Self) {
         *raw = Self::from_bits(value.to_bits().to_le())
     }
@@ -78,10 +80,12 @@ impl Primitive for f32 {
 impl Primitive for f64 {
     type Raw = Self;
 
+    #[inline]
     fn get(raw: &Self::Raw) -> Self {
         Self::from_bits(raw.to_bits().to_le())
     }
 
+    #[inline]
     fn set(raw: &mut Self::Raw, value: Self) {
         *raw = Self::from_bits(value.to_bits().to_le())
     }