First version of sparse strings code.
Get sparse LED string to work with Larson scanner demo (chasers.hpp).
Combine sparse and memory mapped without ifdef and add some comments
Add comments. Clean up code.
Adapt other demos for low ram constraints.
reset code to working chaser demo.
Shrink non-sparse driver code.
Merge remote-tracking branch 'git/sparse_strings'
Merge branch 'sparse_strings'