integrate 'construct' python library

'construct' is a declarative symmetric encoder/decoder for user
specified binary formats.  It should come in extremely handy in
tools like pySim.

We start the integration by adding transport methods for transceiving
APDUs with built-in encoding of the command data and decoding of the
response data.

Change-Id: Ibf457aa8b9480a8db5979defcfafd67674303f6c

pySim/construct.py

@@ -0,0 +1,42 @@
+from construct import *
+from pySim.utils import b2h, h2b
+
+"""Utility code related to the integration of the 'construct' declarative parser."""
+
+# (C) 2021 by Harald Welte <laforge@osmocom.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+class HexAdapter(Adapter):
+    """convert a bytes() type to a string of hex nibbles."""
+    def _decode(self, obj, context, path):
+        return b2h(obj)
+    def _encode(self, obj, context, path):
+        return h2b(obj)
+
+def filter_dict(d, exclude_prefix='_'):
+    """filter the input dict to ensure no keys starting with 'exclude_prefix' remain."""
+    res = {}
+    for (key, value) in d.items():
+        if key.startswith(exclude_prefix):
+            continue
+        if type(value) is dict:
+            res[key] = filter_dict(value)
+        else:
+            res[key] = value
+    return res
+
+# here we collect some shared / common definitions of data types
+LV = Prefixed(Int8ub, HexAdapter(GreedyBytes))