refactor(header): inline FNV hasher to reduce dependencies

Cargo.toml

@@ -37,7 +37,6 @@ std = []
 
 [dependencies]
 bytes = "1"
-fnv = "1.0.5"
 itoa = "1"
 
 [dev-dependencies]

src/header/map.rs

@@ -3654,8 +3654,6 @@ fn hash_elem_using<K>(danger: &Danger, k: &K) -> HashValue
 where
     K: Hash + ?Sized,
 {
-    use fnv::FnvHasher;
-
     const MASK: u64 = (MAX_SIZE as u64) - 1;
 
     let hash = match *danger {
@@ -3667,7 +3665,7 @@ where
         }
         // Fast hash
         _ => {
-            let mut h = FnvHasher::default();
+            let mut h = FnvHasher::new();
             k.hash(&mut h);
             h.finish()
         }
@@ -3676,6 +3674,32 @@ where
     HashValue((hash & MASK) as u16)
 }
 
+struct FnvHasher(u64);
+
+impl FnvHasher {
+    #[inline]
+    fn new() -> Self {
+        FnvHasher(0xcbf29ce484222325)
+    }
+}
+
+impl std::hash::Hasher for FnvHasher {
+    #[inline]
+    fn finish(&self) -> u64 {
+        self.0
+    }
+
+    #[inline]
+    fn write(&mut self, bytes: &[u8]) {
+        let mut hash = self.0;
+        for &b in bytes {
+            hash = hash ^ (b as u64);
+            hash = hash.wrapping_mul(0x100000001b3);
+        }
+        self.0 = hash;
+    }
+}
+
 /*
  *
  * ===== impl IntoHeaderName / AsHeaderName =====