0.6.0 Feature Release

Primarily introduces fixed size arrays in structs and a primary attribute for sorting buffers recursively:

Release 0.6.0 introduces a "primary" attribute to be used together with a key attribute to chose default key for finding and sorting. If primary is absent, the key with the lowest id becomes primary. Tables and vectors can now be sorted recursively on primary keys. BREAKING: previously the first listed, not the lowest id, would be the primary key. Also introduces fixed size scalar arrays in struct fields (struct and enum elements are not supported). Structs support fixed size array fields, including char arrays. Empty structs never fully worked and are no longer supported, they are also no longer supported by flatc. NOTE: char arrays are not currently part of Googles flatc compiler - int8 arrays may be used instead. BREAKING: empty structs are no longer supported - they are also not valid in Googles flatc compiler. See CHANGELOG for additional changes. DEPRECATED: low-level cast_to/from from functions in flatcc_accessors.h will be removed in favor of read/write_from/to because the cast interface breaks float conversion on some uncommon platforms. This should not affect normal use but remains valid in this release.