Resolve get_first clippy lint

    error: accessing first element with `bytes.get(0)`
       --> src/parse.rs:265:8
        |
    265 |     if bytes.get(0) == Some(&b'=') {
        |        ^^^^^^^^^^^^ help: try: `bytes.first()`
        |
        = note: `-D clippy::get-first` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

    error: accessing first element with `bytes.get(0)`
       --> src/parse.rs:267:15
        |
    267 |     } else if bytes.get(0) == Some(&b'>') {
        |               ^^^^^^^^^^^^ help: try: `bytes.first()`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

    error: accessing first element with `bytes.get(0)`
       --> src/parse.rs:273:15
        |
    273 |     } else if bytes.get(0) == Some(&b'<') {
        |               ^^^^^^^^^^^^ help: try: `bytes.first()`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

    error: accessing first element with `bytes.get(0)`
       --> src/parse.rs:279:15
        |
    279 |     } else if bytes.get(0) == Some(&b'~') {
        |               ^^^^^^^^^^^^ help: try: `bytes.first()`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

    error: accessing first element with `bytes.get(0)`
       --> src/parse.rs:281:15
        |
    281 |     } else if bytes.get(0) == Some(&b'^') {
        |               ^^^^^^^^^^^^ help: try: `bytes.first()`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

src/parse.rs

@@ -262,23 +262,23 @@ fn identifier(input: &str, pos: Position) -> Result<(&str, &str), Error> {
 
 fn op(input: &str) -> (Op, &str) {
     let bytes = input.as_bytes();
-    if bytes.get(0) == Some(&b'=') {
+    if bytes.first() == Some(&b'=') {
         (Op::Exact, &input[1..])
-    } else if bytes.get(0) == Some(&b'>') {
+    } else if bytes.first() == Some(&b'>') {
         if bytes.get(1) == Some(&b'=') {
             (Op::GreaterEq, &input[2..])
         } else {
             (Op::Greater, &input[1..])
         }
-    } else if bytes.get(0) == Some(&b'<') {
+    } else if bytes.first() == Some(&b'<') {
         if bytes.get(1) == Some(&b'=') {
             (Op::LessEq, &input[2..])
         } else {
             (Op::Less, &input[1..])
         }
-    } else if bytes.get(0) == Some(&b'~') {
+    } else if bytes.first() == Some(&b'~') {
         (Op::Tilde, &input[1..])
-    } else if bytes.get(0) == Some(&b'^') {
+    } else if bytes.first() == Some(&b'^') {
         (Op::Caret, &input[1..])
     } else {
         (Op::DEFAULT, input)