libutil: More efficient readLittleEndian

[xokdvium]

Motivation

Because unsigned char * can alias anything, the compiler has no choice but to actually iterate byte-by-byte in readNum and readLittleEndian:

        │      movzbl -0x2f(%rbp),%eax
  12.54 │      movzbl -0x2e(%rbp),%edx
        │      shl    $0x8,%rax
   1.88 │      shl    $0x10,%rdx
        │      or     %rdx,%rax
        │      movzbl -0x30(%rbp),%edx
   5.09 │      or     %rdx,%rax
   2.37 │      movzbl -0x2d(%rbp),%edx
        │      shl    $0x18,%rdx
   3.95 │      or     %rdx,%rax
        │      movzbl -0x2c(%rbp),%edx
        │      shl    $0x20,%rdx
        │      or     %rax,%rdx
   5.59 │      movzbl -0x2b(%rbp),%eax
   3.29 │      shl    $0x28,%rax
        │      or     %rdx,%rax
   7.83 │      movzbl -0x2a(%rbp),%edx
        │      shl    $0x30,%rdx
        │      or     %rax,%rdx
   8.22 │      movzbl -0x29(%rbp),%eax
        │      shl    $0x38,%rax
        │      or     %rdx,%rax
   6.42 │      mov    %rax,%rcx
        │      mov    %rax,-0x60(%rbp)
   1.35 │      shr    $0x20,%rcxA

Which now compiles down to:

   2.20 │      mov  -0x30(%rbp),%rax
   3.12 │      mov  %rax,%rcx
        │      mov  %rax,-0x60(%rbp)
        │      shr  $0x20,%rcx

Also adds some tests for cases for serialisation/de-serialisation of negative values that get converted to uint64_t.

Context

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[xokdvium]

We could use https://en.cppreference.com/w/cpp/numeric/byteswap.html here if macOS libc++ wasn't so retarted.