Remove legacy arbitrary precision integer support:

The CBigNum class is a wrapper around OpenSSL's BIGNUM implementation
to make use simpler.

Replacing the implementation with boost::multiprecision helps reduce
the size of the codebase and improves performance (benchmarks show
the new boost-based implementation is ~7x faster).

src/ripple/protocol/STAmount.h

@@ -171,8 +171,6 @@ public:
     STAmount
     zeroed() const
     {
-        // TODO(tom): what does this next comment mean here?
-        // See https://ripplelabs.atlassian.net/browse/WC-1847?jql=
         return STAmount (mIssue);
     }