Select a new case table for the current buffer.
A case table is a char-table which maps characters
to their lower-case equivalents. It also has three "extra" slots
which may be additional char-tables or nil.
These slots are called UPCASE, CANONICALIZE and EQUIVALENCES.
UPCASE maps each non-upper-case character to its upper-case equivalent.
(The value in UPCASE for an upper-case character is never used.)
If lower and upper case characters are in 1-1 correspondence,
you may use nil and the upcase table will be deduced from DOWNCASE.
CANONICALIZE maps each character to a canonical equivalent;
any two characters that are related by case-conversion have the same
canonical equivalent character; it may be nil, in which case it is
deduced from DOWNCASE and UPCASE.
EQUIVALENCES is a map that cyclically permutes each equivalence class
(of characters with the same canonical equivalent); it may be nil,
in which case it is deduced from CANONICALIZE. (fn TABLE)