codec/encoder: rename set_rate() to set_bit_rate()

This commit serves the purpose to remove ambiguity:
- The decoder actually calls the getter for this field `bit_rate()`
- The audio decoder/encoder have methods called `{set_}rate()` for the _rate`
- The encoder/audio `set_rate()` shadows the encoder/mod `set_rate()`

src/codec/encoder/mod.rs

@@ -58,7 +58,7 @@ impl Encoder {
 		}
 	}
 
-	pub fn set_rate(&mut self, value: usize) {
+	pub fn set_bit_rate(&mut self, value: usize) {
 		unsafe {
 			(*self.as_mut_ptr()).bit_rate = value as c_int;
 		}