LibCompress: Decode non-self-referencing back-references in one shot

We currently decode back-references one byte at a time, while writing
that byte back out to the output buffer. This is only necessary when the
back-reference refers to itself, i.e. when the back-reference distance
is less than its length. In other cases, we can read the entire back-
reference block in one shot.

Using the "enwik8" file as a test (100MB uncompressed, commonly used in
benchmarks: https://www.mattmahoney.net/dc/enwik8.zip), decompression
time decreases from:

    5.8s to 4.89s on Serenity (cold)
    2.3s to 1.72s on Serenity (warm)
    1.6s to 1.06s on Linux

Userland/Libraries/LibCompress/Deflate.h

@@ -104,6 +104,8 @@ private:
     ErrorOr<u32> decode_distance(u32);
     ErrorOr<void> decode_codes(CanonicalCode& literal_code, Optional<CanonicalCode>& distance_code);
 
+    static constexpr u16 max_back_reference_length = 258;
+
     bool m_read_final_bock { false };
 
     State m_state { State::Idle };