added unit test for required_fields_bitmap hashing issue
t/: rename the required fields bitmap test case
protobuf_c_message_unpack(): revert hash-based required field detection

Originally, someone complained about protobuf_c_message_unpack() using
alloca() for the allocation of the temporary bitmap used to detect that
all required fields were present in the unpacked message (Issue #60).
Commit 248eae1d eliminated the use of alloca(), replacing the
variable-length alloca()'d bitmap with a 16 byte stack-allocated bitmap,
treating field numbers mod 128.

Andrei Nigmatulin noted in PR #137 problems with this approach:

    Apparently 248eae1d has introduced a serious problem to protobuf-c
    decoder.

    Originally the function of required_fields_bitmap was to prevent
    decoder from returning incomplete messages. That means, each
    required field in the message either must have a default_value or be
    present in the protobuf stream. The purpose of this behaviour was to
    provide user with 100% complete ProtobufCMessage struct on return
    from protobuf_c_message_unpack(), which does not need to be checked
    for completeness just after. This is exactly how original protobuf
    C++ decoder behaves. The patch 248eae1d broke this functionality by
    hashing bits of required fields instead of storing them separately.

    Consider a protobuf message with 129 fields where the first and the
    last fields set as 'required'. In this case it is possible to trick
    decoder to return incomplete ProtobufCMessage struct with missing
    required fields by providing only one of the two fields in the
    source byte stream. This can be considered as a security issue as
    well because user's code do not expect incomplete messages with
    missing required fields out from protobuf_c_message_unpack(). Such a
    change could introduce undefined behaviour to user programs.

This patch is based on Andrei's fix and restores the exact detection of
missing required fields, but avoids doing a separate allocation for the
required fields bitmap except for messages whose descriptors define a
large number of fields. In the "typical" case where the message
descriptor has <= 128 fields we can just use a 16 byte array allocated
on the stack. (Note that the hash-based approach also used a 16 byte
stack allocated array.)
ChangeLog: credit Andrei Nigmatulin for #136, #137
.travis.yml: test on both gcc and clang