Add _DynamicStructBuilder.to_bytes() and <struct module>.read_from_memory

to_bytes is not very efficient (it makes one necessary copy and one
unnecessary copy), but it will simplify a lot of use cases.

read_from_memory should be maximally efficient for most input types
(including mmap).

capnp/capnp_cpp.pxd

@@ -11,6 +11,8 @@ from libcpp cimport bool as cbool
 cdef extern from "capnp/common.h" namespace " ::capnp":
     enum Void:
         VOID " ::capnp::VOID"
+    cdef cppclass word:
+        pass
 
 cdef extern from "kj/string.h" namespace " ::kj":
     cdef cppclass StringPtr:
@@ -31,6 +33,23 @@ cdef extern from "kj/common.h" namespace " ::kj":
         size_t size()
         T& operator[](size_t index)
 
+    # Cython can't handle ArrayPtr[word] as a function argument
+    cdef cppclass WordArrayPtr "::kj::ArrayPtr<::capnp::word>":
+        WordArrayPtr()
+        WordArrayPtr(word *, size_t size)
+        size_t size()
+        word& operator[](size_t index)
+
+cdef extern from "kj/array.h" namespace " ::kj":
+    cdef cppclass Array[T]:
+        T* begin()
+        size_t size()
+
+    # Cython can't handle Array[word] as a function argument
+    cdef cppclass WordArray "::kj::Array<::capnp::word>":
+        word* begin()
+        size_t size()
+
 cdef extern from "capnp/schema.h" namespace " ::capnp":
     cdef cppclass Schema:
         Node.Reader getProto() except +