Take keys by reference

[diondokter]

Fixes #58

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
 16700	     56	  33808	  50564	   c584	example

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +55%     +21  [ = ]       0    [Unmapped]
  -0.3%     -32  [ = ]       0    .debug_aranges
  -0.5%    -112  [ = ]       0    .debug_frame
  -2.4%    -192  [ = ]       0    .symtab
  -0.3%    -389  [ = ]       0    .debug_loc
  -0.9%    -472  [ = ]       0    .debug_ranges
  -3.0%    -504  -3.0%    -504    .text
  -0.7%    -540  [ = ]       0    .debug_line
  -7.6% -1.39Ki  [ = ]       0    .strtab
  -0.7% -2.30Ki  [ = ]       0    .debug_info
  -0.9% -2.40Ki  [ = ]       0    .debug_str
  -0.9% -8.25Ki  -1.0%    -504    TOTAL

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
 16700	     56	  33808	  50564	   c584	example

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +61%     +23  [ = ]       0    [Unmapped]
  -0.3%     -32  [ = ]       0    .debug_aranges
  -0.5%    -112  [ = ]       0    .debug_frame
  -2.4%    -192  [ = ]       0    .symtab
  -0.3%    -389  [ = ]       0    .debug_loc
  -0.9%    -472  [ = ]       0    .debug_ranges
  -3.0%    -504  -3.0%    -504    .text
  -0.7%    -554  [ = ]       0    .debug_line
  -7.6% -1.39Ki  [ = ]       0    .strtab
  -0.7% -2.32Ki  [ = ]       0    .debug_info
  -0.9% -2.40Ki  [ = ]       0    .debug_str
  -0.9% -8.29Ki  -1.0%    -504    TOTAL