v0.4.0: AST Traversal

The focus of this release was the creation of tools to traverse and manipulate the JSON AST.

• Creates a generic visitor system for jsonv::value
• Creates the path system, which is a very simplified version of JSONPath
• Creates the merge and traverse families of algorithms
• Creates the coerce library for non-strict conversion
• Extends array_view and object_view to have owning versions, so calling value::as_array and
  value::as_object on rvalues works as you would expect (safely)
• Make kind::decimal and kind::integer equivalent in almost all cases