Make REPL allow user-defined parsers

[myreen]

At the moment the CakeML/Candle REPL is user configurable through the following references.

cakeml/compiler/repl/repl_moduleProgScript.sml

Lines 10 to 20 in 77cbb88

	This Repl module defines some references:
	- Repl.isEOF : bool ref
	-- true means that all user input has been read (e.g. if we have
	reached the end of stdin)
	- Repl.nextString : string ref
	-- contains the next user input (if isEOF is false)
	- Repl.readNextString : (unit -> unit) ref
	-- the function that the Repl uses to read user input; it is this
	function that assigns new values to Repl.isEOF and Repl.nextString
	- Repl.exn : exn
	-- the most recent exception to propagate to the top

Repl.readNextString holds a function that populates Repl.isEOF : bool ref and Repl.nextString : string ref.

This issue is about:

• changing the type of Repl.nextString to (string + Ast.dec list) ref
• moving the OCaml parser from the bootstrap translation to the candle boot files

In the new setting, the CakeML/Candle REPL starts by reading the initial input from a boot file written in CakeML concrete syntax. This file can be used to define a new parser, e.g., an OCaml parser. The new parser can then be put to use by updating Repl.readNextString : (unit -> unit) ref so that it parses input using the new parser and assigns it to the Ast.dec list option of Repl.nextString.

Perhaps Repl.nextString should be renamed to Repl.nextInput.