Support user-defined exceptions in Lem shallow embedding

The type-checker already supports a user-defined "exception" type that can be
used in throw and try-catch expressions.  This patch adds support for that to
the Lem shallow embedding by adapting the existing exception mechanisms of the
state and prompt monads.  User-defined exceptions are distinguished from
builtin exception cases.  For example, the state monad uses

type ex 'e =
  | Exit
  | Assert of string
  | Throw of 'e

to distinguish between calls to "exit", failed assertions, and user-defined
exceptions, respectively.  Early return is also handled using the exception
mechanism, by lifting to a monad with "either 'r exception" as the exception
type, where 'r is the expected return type and "exception" is the user-defined
exception type.

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