Drop unsafe code from currently unsound function prefix()

[Shnatsel]

Fixes #29

This changes the generated assembly from

 movzx   ecx, byte, ptr, [rdi]
 movzx   eax, byte, ptr, [rdi, +, 1]
 movzx   edx, byte, ptr, [rdi, +, 2]
 shl     edx, 16
 shl     eax, 8
 or      eax, ecx
 or      eax, edx
 ret

to

 push    rax
 cmp     rsi, 2
 jbe     .LBB106_1
 movzx   ecx, byte, ptr, [rdi]
 movzx   eax, byte, ptr, [rdi, +, 1]
 movzx   edx, byte, ptr, [rdi, +, 2]
 shl     edx, 16
 shl     eax, 8
 or      eax, ecx
 or      eax, edx
 pop     rcx
 ret
.LBB106_1:
 mov     edi, 3
 call    qword, ptr, [rip, +, _ZN4core5slice20slice_index_len_fail17hbeb3bb6238e26ddbE@GOTPCREL]
 ud2

which is as good as it's going to get without restructuring the code.

This change incurs at about 1% performance penalty. This was tested without inlining, which may elide bounds checks further. Performance penalty can probably be avoided altogether by using slice::windows() plus one of the ways to skip iterator elements in function flush() instead of indexing, but I'm not sure it's worth the trouble.

[Shnatsel]

FWIW I've also tried the following:

    let mut result = [0, 0, 0];
    result.copy_from_slice(&input_buf[..3]);
    result

which yielded the exact same assembly.

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   91.44%   91.46%   +0.01%     
==========================================
  Files          14       14              
  Lines        1297     1300       +3     
==========================================
+ Hits         1186     1189       +3     
  Misses        111      111

Impacted Files	Coverage Δ
src/lz77/default.rs	89.1% <100%> (-0.22%)	⬇️
src/deflate/encode.rs	96.62% <0%> (+0.02%)	⬆️
src/deflate/symbol.rs	93.06% <0%> (+0.02%)	⬆️
src/huffman.rs	94% <0%> (+0.06%)	⬆️
src/non_blocking/deflate/decode.rs	86.85% <0%> (+0.07%)	⬆️

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[sile]

LGTM. Thanks!
I think 1% performance penalty is acceptable.