Merge multiprecision from Trunk.

svn path=/branches/release/boost/multiprecision/; revision=81806
Merge minor fixes from Trunk.

svn path=/branches/release/boost/multiprecision/; revision=81879
Merge bug fixes from Trunk:
Mostly correct typos as per https://svn.boost.org/trac/boost/ticket/7806
Also fix cpp_dec_float docs and add some static asserts to catch misuse.
Fix performance bottleneck in cpp_int's gcd implementation.

svn path=/branches/release/boost/multiprecision/; revision=82122
Fix bug in cpp_int division.
Update docs.
Refs #7878.

svn path=/branches/release/boost/multiprecision/; revision=82497