lib: json-tree - API change to permit more featureful implementation Provide accessors for the innards of the tree's nodes, so that the node internals can be changed freely (needed for DAPI). Constify params where sensible. NOTE: this enforced const-correctness may *break* (cause a failure to build) some clients that didn't use const where they could have. Only obox2 is known to break. Add sibling functions with more parameters to expand the range of functionality, and inline rewire the old simple functions to call the more generic ones. Signed-off-by: Phil Carmody <phil@dovecot.fi>

lib: Added json-tree API for parsing JSON input into a tree structure. This makes it easier to access complicated JSON structs that can fit into memory.