Update thread from develop.
Remove release directory from main tree.

Now at: https://github.com/boostorg/release/
Update math from develop.
Update type_traits from develop.
Update phoenix from develop.
Update smart_ptr from develop.
Update phoenix, geometry from develop.
Update phoenix from develop.
Update phoenix from develop.
Update phoenix from develop.
Update phoenix from develop.
Update phoenix from develop.
Update signals from develop.
Update serialization from develop.
Update multiprecision from develop.
Update multiprecision from develop.
Update phoenix from develop.