- Caching of char<->byte conversions in strings - should make nearly
  all operations on strings O(N)

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String access speedup:
----------------------

For strings larger than some size in bytes (10?), keep extra fields of
info: length in chars, and a (char, byte) pair in the middle to speed
up sequential access.

(Better idea: do this for any size string, but only if it contains
non-ASCII chars.  Then if info is missing, we know string is
ASCII-only.)

Use a string-extra-info object, replacing string property slot and
containing fields for string mod tick, string extents, string props,
and string char length, and cached (char,byte) pair.
string-extra-info (or string-auxiliary?) objects could be in frob
blocks, esp. if creating frob blocks is easy + worth it.