Hide internals (dispatch_init(), init_from_block_range(), m_append())

[jeking3]

Details: #67

There are a few methods that do not need to be public, perhaps, so according to @glenfe :

• In Boost 1.80.0, announce in documentation and in release notes that these functions will only be provisionally public in Boost 1.81.0 if macros are defined.
• In Boost 1.81.0 these functions will only be public if some macro like BOOST_DYNAMIC_BITSET_DEPRECATED_PUBLIC is defined. The release notes and documentation will state that in Boost 1.83.0 these functions will be unconditionally private.
• In Boost 1.83.0 the functions will be unconditionally private.