src/trezor/crypto: introduce SecureContext

usage:

from trezor.crypto import SecureContext

with SecureContext() as sc:
    sc.var1 = ...
    sc.var2 = ...

SecureContext will call destructors of all variables assigned
to sc in the block. It will also call gc.collect()

src/trezor/crypto/__init__.py

@@ -1,3 +1,5 @@
+from gc import collect
+
 from trezorcrypto import (  # noqa: F401
     bip32,
     bip39,
@@ -8,3 +10,18 @@ from trezorcrypto import (  # noqa: F401
     random,
     rfc6979,
 )
+
+
+class SecureContext:
+    def __init__(self):
+        pass
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        for k in self.__dict__:
+            o = getattr(self, k)
+            if hasattr(o, "__del__"):
+                o.__del__()
+        collect()