Improve performance of decoding

[taiki-e]

This change makes decoding about 10x faster than the current implementation, on my machine.

Before:

hex_decode              time:   [128.28 us 129.41 us 130.70 us]

After:

hex_decode              time:   [12.206 us 12.242 us 12.284 us]                        

Benchmarks per commit

Current main branch (aa8f300)

hex_decode              time:   [128.28 us 129.41 us 130.70 us]

First commit of this PR: Use lookup table for decoding (870590e)

hex_decode              time:   [63.538 us 63.936 us 64.419 us]
                        change: [-50.833% -50.462% -50.088%] (p = 0.00 < 0.05)
                        Performance has improved.

Second commit of this PR: Inline val function (3b75b32)

hex_decode              time:   [32.263 us 32.382 us 32.521 us]
                        change: [-49.441% -49.098% -48.682%] (p = 0.00 < 0.05)
                        Performance has improved.

Third commit of this PR: Use chunks_exact instead of chunks (ff1d115)

hex_decode              time:   [24.701 us 24.814 us 24.936 us]
                        change: [-24.459% -23.911% -23.395%] (p = 0.00 < 0.05)
                        Performance has improved.

Fourth commit of this PR: Use decode_to_slice in Vec::from_hex (4fd0ed8)

hex_decode              time:   [16.060 us 16.297 us 16.608 us]
                        change: [-35.137% -34.468% -33.556%] (p = 0.00 < 0.05)
                        Performance has improved.

Fifth commit of this PR: Inline decode_to_slice function (3e2aa25)

hex_decode              time:   [12.206 us 12.242 us 12.284 us]                        
                        change: [-21.954% -20.254% -18.150%] (p = 0.00 < 0.05)
                        Performance has improved.

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[Luro02]

Looks good :)