std: optimise utf8ByteSequenceLength

Also tested (but not as fast):
```zig
pub fn utf8ByteSequenceLength(first_byte: u8) !u3 {
    const len = @clz(u8, ~first_byte);
    if (len == 0) return 1;
    if (len < 4) return @intcast(u3, len);
    return error.Utf8InvalidStartByte;
}
```

Author: daurnimator

lib/std/unicode.zig

@@ -18,11 +18,13 @@ pub fn utf8CodepointSequenceLength(c: u32) !u3 {
 /// returns a number 1-4 indicating the total length of the codepoint in bytes.
 /// If this byte does not match the form of a UTF-8 start byte, returns Utf8InvalidStartByte.
 pub fn utf8ByteSequenceLength(first_byte: u8) !u3 {
-    if (first_byte < 0b10000000) return @as(u3, 1);
-    if (first_byte & 0b11100000 == 0b11000000) return @as(u3, 2);
-    if (first_byte & 0b11110000 == 0b11100000) return @as(u3, 3);
-    if (first_byte & 0b11111000 == 0b11110000) return @as(u3, 4);
-    return error.Utf8InvalidStartByte;
+    return switch (@clz(u8, ~first_byte)) {
+        0 => 1,
+        2 => 2,
+        3 => 3,
+        4 => 4,
+        else => error.Utf8InvalidStartByte,
+    };
 }
 
 /// Encodes the given codepoint into a UTF-8 byte sequence.