Add code to parse terms from strings rather than from streams.

Estimated hours taken: 24

Add code to parse terms from strings rather than from streams.
The original intention for this change was twofold:
to increase expressiveness and to improve efficiency.
However, for the moment I have given up on the goal of
improving efficiency.

library/io.m:
	Add io__read_file_as_string/{4,5}, for efficiently
	reading a whole file into a single string.
	Add io__read_from_string/5, for reading terms of
	any type from a string.
	(Should that instead be named string__read instead?)

library/string.m:
	Implement string__substring/4 more efficiently,
	using `pragma c_code' rather than by calling
	string__right(string__left(...)).
	Export string__unsafe_index/3, and add new exported
	predicate string__unsafe_substring/4 -- these
	are versions of string__index and string__substring
	that don't check for range errors.  They are
	needed to get reasonable efficiency when dealing
	with very large strings.

library/string.nu.nl:
	Add Prolog implementations of string__substring
	and string__unsafe_substring.

library/lexer.m:
	Add lexer__string_get_token_list/{5,6}, for parsing
	tokens from a string.  This required adding
	`string_' versions of most of the lexical analysis routines.
	XXX lots of code duplication, I'm afraid.
	XXX the string versions are not as efficiency as they
	    could ideally be.

library/parser.m:
	Add new predicates parser__read_term_from_string/{4,6}
	and parser__parse_token_list.

compiler/bytecode_gen.m:
compiler/error_util.m:
compiler/fact_table.m:
compiler/term_errors.m:
	Minor changes to use term__context as an ADT rather than
	as a concrete data type.

In addition, I made the following changes, which I am NOT committing.

compiler/prog_io.m:
	Change to use io__read_file_as_string and
	parser__read_term_from_string.
	(Not committed because the existing method is
	in currently more efficient.)

library/term.m:
	Add new alternative term__detailed_context to the term__context type.
	This alternative can record both the start and end of a term rather
	than just the end.
	(The lexer now has almost all the necessary support for detailed
	contexts, but enabling it would have some potentially significant
	efficiency costs, and curently the parser uses only single contexts
	rather than ranges; so currently this is not used.)

compiler/term_errors.m

@@ -544,7 +544,8 @@ term_errors__describe_several_proc_names([PPId | PPIds], Module,
 
 term_errors__describe_one_call_site(PPId - Context, Module, Piece) :-
 	term_errors__describe_one_proc_name(PPId, Module, ProcName),
-	Context = term__context(FileName, LineNumber),
+	term__context_file(Context, FileName),
+	term__context_line(Context, LineNumber),
 	string__int_to_string(LineNumber, LineNumberPart),
 	string__append_list([
 		ProcName,