Rather than having a separate variable for each backend X,
opt_print_X, just have a single variable opt_print_target, where
target contains a string option, such as `Some "lem"` or `Some
"ocaml"`, then we have a function target that takes that string and
invokes the appropriate backend, so the main function in sail.ml goes
from being a giant if-then-else block to a single call to

target !opt_target ast env

This allows us to implement a :compile <target> command in the
interactive toplevel

Also implement a :rewrites <target> command which performs all the
rewrites for a specific target, so rather than doing e.g.

> sail -c -O -o out $FILES

one could instead interactively do

> sail -i
:option -undefined_gen
:load $FILES
:option -O
:option -o out
:rewrites c
:compile c
:quit

for the same result.

To support this the behavior of the interactive mode has changed
slightly. It no longer performs any rewrites at all, so a :rewrites
interpreter is currently needed to interpret functions in the
interactive toplevel, nor does it automatically set any other flags,
so -undefined_gen is needed in this case, which is usually implied by
the -c flag.