Further consistency improvements for error and warning messages.

The two main changes in this commit are:

(1) Use the error_util library's decl/1 and pramga_decl/1 format components in more
places.

(2) Quote type variables that appear in error messages.

compiler/error_util.m:
	Add a version of list_to_pieces/1 that places each of the strings in
	the input list in single quotes.

compiler/prog_io_type_defn.m:
	Quote type variables names in error messages.

compiler/add_class.m:
compiler/add_pragma.m:
compiler/add_pred.m:
compiler/check_typeclass.m:
compiler/make_hlds_error.m:
compiler/post_typecheck.m:
compiler/prog_io.m:
	Use the error_util library's decl/1 and pragma_decl/1 format components
	in more places.

compiler/mode_errors.m:
	Quote program variables and insts in some error messages.

compiler/post_term_analysis.m:
	Format a type ctor and arity in an error message properly.

tests/invalid/*.err_exp:
tests/warnings/*.exp:
	Conform to the above changes.