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.cpp

@@ -199,12 +199,18 @@ ErrorOr<bool> DeflateDecompressor::CompressedBlock::try_read_more()
 
         auto const distance = TRY(m_decompressor.decode_distance(distance_symbol));
 
+        if (distance < length) {
             for (size_t idx = 0; idx < length; ++idx) {
                 u8 byte = 0;
                 TRY(m_decompressor.m_output_buffer.read_with_seekback({ &byte, sizeof(byte) }, distance));
 
                 m_decompressor.m_output_buffer.write({ &byte, sizeof(byte) });
             }
+        } else {
+            Array<u8, DeflateDecompressor::max_back_reference_length> buffer;
+            auto bytes = TRY(m_decompressor.m_output_buffer.read_with_seekback({ buffer.data(), length }, distance));
+            m_decompressor.m_output_buffer.write(bytes);
+        }
 
         return true;
     }
@@ -386,7 +392,7 @@ ErrorOr<u32> DeflateDecompressor::decode_length(u32 symbol)
     }
 
     if (symbol == 285)
-        return 258;
+        return DeflateDecompressor::max_back_reference_length;
 
     VERIFY_NOT_REACHED();
 }

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 };