fix: check field type via C++ schema in from_dict to avoid a reference cycle (#407)

from_dict called self.schema.fields for str values, which builds
_StructSchemaField wrappers that reference the schema, so every
message built via kwargs stayed alive until a GC pass. Follow-up to
#401, which only skipped the lookup for non-str values.

capnp/includes/capnp_cpp.pxd

@@ -128,6 +128,7 @@ cdef extern from "capnp/schema.h" namespace " ::capnp":
         cbool isEnum()
         cbool isStruct()
         cbool isInterface()
+        cbool isData()
 
         StructSchema asStruct() except +reraise_kj_exception
         EnumSchema asEnum() except +reraise_kj_exception
@@ -185,7 +186,7 @@ cdef extern from "capnp/schema.h" namespace " ::capnp":
         FieldSubset getUnionFields()
         FieldSubset getNonUnionFields()
 
-        Field getFieldByName(char * name)
+        Field getFieldByName(char * name) except +reraise_kj_exception
 
         cbool operator == (StructSchema)