Make keys dynamic length

[diondokter]

Fixes #39
Fixes #46

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
 17176	     56	  33808	  51040	   c760	example

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.6% +1.67Ki  [ = ]       0    .debug_str
  +0.2%    +694  [ = ]       0    .debug_info
  +0.3%    +128  [ = ]       0    .debug_ranges
  +0.2%     +33  [ = ]       0    .strtab
  +0.4%     +32  [ = ]       0    .symtab
   +44%     +20  [ = ]       0    [Unmapped]
  +0.1%     +16  [ = ]       0    .debug_frame
  +0.1%      +8  [ = ]       0    .debug_aranges
  -1.0%     -42  [ = ]       0    .debug_abbrev
  -0.3%     -52  -0.3%     -52    .text
  -0.1%     -63  [ = ]       0    .debug_line
  -0.5%    -783  [ = ]       0    .debug_loc
  +0.2% +1.66Ki  -0.1%     -52    TOTAL

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
 17176	     56	  33808	  51040	   c760	example

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.6% +1.67Ki  [ = ]       0    .debug_str
  +0.2%    +694  [ = ]       0    .debug_info
  +0.3%    +128  [ = ]       0    .debug_ranges
  +0.2%     +33  [ = ]       0    .strtab
  +0.4%     +32  [ = ]       0    .symtab
   +44%     +20  [ = ]       0    [Unmapped]
  +0.1%     +16  [ = ]       0    .debug_frame
  +0.1%      +8  [ = ]       0    .debug_aranges
  -1.0%     -42  [ = ]       0    .debug_abbrev
  -0.3%     -52  -0.3%     -52    .text
  -0.1%     -63  [ = ]       0    .debug_line
  -0.5%    -783  [ = ]       0    .debug_loc
  +0.2% +1.66Ki  -0.1%     -52    TOTAL