Refactor the common buffer manipulation in rdataslab.c in macros

The rdataslab.c was full of code like this:

    length = raw[0] * 256 + raw[1];

and

    count2 = *current2++ * 256;
    count2 += *current2++;

Refactor code like this into peek_uint16() and get_uint16 macros to prevent code repetition and possible mistakes when copy and pasting the same code over and over.

As a side note for an entertainment of a careful reader of the commit messages: The byte manipulation was changed from multiplication and addition to shift with or.

The difference in the assembly looks like this:

MUL and ADD:

movzx   eax, BYTE PTR [rdi]
    movzx   edi, BYTE PTR [rdi+1]
    sal     eax, 8
    or      edi, eax

SHIFT and OR:

    movzx   edi, WORD PTR [rdi]
    rol     di, 8
    movzx   edi, di

If the result and/or buffer is then being used after the macro call, there's more differences in favor of the SHIFT+OR solution.