doc-src/Tutorial/CodeGen/expr

'split' method: '(asm)' option;
added 'slow', 'slowsimp', 'bestsimp' methods;

types 'v binop = 'v => 'v => 'v
datatype ('a,'v) expr = Cex 'v
                      | Vex 'a
                      | Bex ('v binop)  (('a,'v) expr) (('a,'v) expr)