Writing out the pointer value of unwritable types.
Also reset the context env when updating interaction-environment.
Fixes issue #200.
Fixing bug in the non-strict top-level identifier matching logic.
Matching worked when both bindings were missing, and when both
bindings were present at the top-level but different.  The case
when only one top-level binding was present wasn't handled correctly.
Local lexical matching remains unchanged.
Fixes issue #198.
Adding missing (c-system-include "sys/file.h") for flock constants.
Fixes issue #199.