Changes

Added support for trailing comma for json_path!
Fixed json_path! not using full type path when creating empty array
(could probably have lead to compilation errors if JsonPathPiece had not been imported)
Fixed UnexpectedStructureKind::to_string not including field values
Added JsonReader::seek_back as opposite of seek_to (#53)

Simple API

(Breaking) Removed structs ArrayItemReader and MemberValueReader in favor of the new common struct SingleValueReader
Reader and writer will keep returning errors after the first error (#55)
This is intended as safeguard in case an error is accidentally not propagated by a closure or function provided by the user. However, users should not rely on this behavior, and instead use Rust's ? operator for propagating reader and writer errors.
New ValueReader methods:
- read_str for reading a JSON string value as borrowed str
- read_seeked and read_seeked_multi for making it easier to seek to values and read them (#48, #49)
  Especially read_seeked_multi can be quite useful for reading for example all values of nested JSON arrays or objects; see its documentation for examples.
- read_string_with_reader for reading large string values in a streaming way (#54)
New ValueWriter methods:
- write_string_with_writer for writing large string values in a streaming way (#54)