Fix to get ConstMapML working under PolyML.

Rework code here to allow more type-specific versions of constants to
map to different data. Thus, we can support APPEND mapping to
listML.APPEND everywhere except on char list -> char list -> char
list, where it can map to stringML.STRCAT.

Added EmitML.emitCAML for exporting to Ocaml. This has required a fair
amount of hackery, none of which should effect emitML. In particular,
CAML constructors must start with an uppercase letter and other values
must not. Also, types can't start with a lowercase letter. Constructors
are added to the constants map with ConstMapML.insert_cons.

(I will eventually check-in the script for generating CAML versions of the
code emitted to src/theoryML.)

Moved EmitML code to a new directory src/emit.

Added a new structure EmitTeX. This allows HOL theories to be printed
as LaTeX source.

Added both of these structures to help/src/Keepers.sml