Currently, for every table of a module, corresponding
key/value sscanf/snprintf functions will be generated.
These functions are mostly used for python API to
facilitate passing keys between python and C++.

It is a known issue that these sscanf/snprintf functions
can consume a lot of system resources esp. memory for
large arrays. Commit 22eae8c6 ("Use late-binding to
finalize snprintf/sscanf") avoids unnecessary code
generation for snprintf/sscanf until they are called.
Even with this commit, however, the overhead can still
be significant for large arrays.

For example, the following large array,
  #define TCP6_RXMIT_BUCKET_BITS 18
  struct tcp6_rxmit_tbl {
    __u64 buckets[1 << TCP6_RXMIT_BUCKET_BITS];
  };
  BPF_ARRAY(rxmit_marking_map, struct tcp6_rxmit_tbl, 1);
is used inside Facebook.
If it is added to examples/cpp/HelloWorld.cpp,
the HelloWorld RSS memory will increase from 7MB to
370MB.

This patch add a BPFModule API and a C++ API to
disable rw_engine sscanf/snprintf function through additional
constructor parameters. rw_engine support for Python is not affected.
Signed-off-by: Yonghong Song <yhs@fb.com>