MPack v0.8

Changes:

• Added mpack_peek_tag()
• Added reader helper functions to expect re-ordered map keys
• Improved documentation and added Pages
• Made node key lookups check for duplicate keys
• Added various UTF-8 checking functions for reader and nodes
• Added support for compiling as C in recent versions of Visual Studio
• Removed mpack_expect_str_alloc() and mpack_expect_utf8_alloc()
• Fixed miscellaneous bugs and improved performance