Create symbol to get the version of this library

[rickmark]

As the library approaches 1.0 we should provide a symbol to ask for the version of libsecp256k1

An example such as what is provided by OpenSSL - https://www.openssl.org/docs/man1.1.0/man3/OPENSSL_VERSION_NUMBER.html

Before the 1.0 release candidate series this can use a value of 0.8 or 0.9 to reflect the non-stable API surface

[rickmark]

Created #914 for this

[real-or-random]

What exactly would you need this for? Some more rationale will help. I guess OpenSSL does that for a reason, but our project is much simpler.

I don't think there's a strong reason not to do this, but maybe what you actually want is some kind of ABI version? Or an assert_version function instead of a get_version function?