Represent keyboard chords as sets instead of lists

This prevents duplicate elements and reflects the fact that order is
not significant.
Rewrite in python3
Transform clusters using Awk
Update theory/system
Update copyright years
Add final -t/-d column to RHS
Remove sed shapes program
Add shape for onset /sp/
Always replace hyphen with implicit schwa
Use proper IPA symbol for the rhotic consonant
Fix nasal consonant ordering

The larger cluster has to come first.
Don’t elongate final ‘ɛ’

Such behaviour is not always desirable; a counter-example might be
the word ‘demonstrate’ which starts with a short ‘ɛ’.
Rearrange function definitions
Adjust sed substitution ordering and comments
Don’t replace final ɪ with ɪə
Add ‘mp’ clusters ‘ntk’ and ‘ntdkg’
Add license notice header
Adjust steno machine symbols and shapes

The left hand thumb keys are now labelled ‘æ’ and ‘ɛ’.
Add implicit schwa and more shapes

Had to use /tdrw/ for the /l/ that follows /pl/ and /bl/ because /td/
following /k/ or /g/ could be /kl/ or /gl/.
Disable key repeat during execution

The trap(1) command ensures it will be re-enabled when the program dies.