mie: replace fixed value system with an extensible attribute interface

under this new system, dialects can define their own custom attributes,
complete with their own print() and parse() callbacks, which can then be
used as values in an op's attribute dictionary.

alongside custom dialect attributes, the former int, float, and string
constant values have been converted to attributes provided by the
arith and builtin dialects respectively. the caches for these attributes
have also been moved from mie_ctx to their respective dialect data
structures.

this system will allow new types of attributes to be implemented,
including dictionaries, arrays, and references to types themselves
(rather than just particular values of a given type).

mie/dialect/select/attribute/graph-scope.c

@@ -0,0 +1,21 @@
+#include <mie/dialect/dialect.h>
+#include <mie/macros.h>
+#include <mie/trait/trait-definition.h>
+#include <mie/trait/trait.h>
+
+static enum mie_status validate(
+	const struct mie_trait_definition *trait_def,
+	const struct mie_trait *trait, const struct mie_trait_target *target)
+{
+	return MIE_SUCCESS;
+}
+
+/* select.graph-scope trait:
+ * regions of an op that has this trait are graph regions. graph regions
+ * cannot have more than one block, and the entry block must be unnamed
+ * with no parameters. However, graph ops can be used. */
+MIE_TRAIT_DEFINITION_BEGIN(mie_select_graph_scope, "graph-scope")
+	MIE_TRAIT_DEFINITION_TARGETS(MIE_TRAIT_TARGET_OP | MIE_TRAIT_TARGET_TYPE);
+	MIE_TRAIT_DEFINITION_STRUCT(struct mie_trait);
+	MIE_TRAIT_DEFINITION_VALIDATE(validate);
+MIE_TRAIT_DEFINITION_END()