PHPC-326, PHPC-324

This PR adds new classes to represent BSON arrays and documents to the MongoDB\BSON namespace. Both classes are backed by a bson_t and are currently read-only. The interfaces for both classes differ a bit, mainly due to constraints in how libbson handles bson_t:

• Document can be constructed from a JSON string, but ArrayList can't as libbson doesn't offer any functionality for that.
• Document has accessors to get canonical and extended JSON strings for a document, but ArrayList doesn't as libbson doesn't support that.
• Both classes have a private constructor, as there's no point in creating an empty instance. This may be relaxed in the future once we allow editing BSON structures.

To iterate a BSON structure, the new Iterator class was introduced. Again, this class can't be instantiated directly (mainly due to ZPP not handling union types properly), but will be returned as part of the above classes' implementation of IteratorAggregate.

To receive BSON instances, the new typeMap option bson was introduced. It can be used on the root level, but also for the document and array types, as well as in field path mappings for specific fields. The most common use case will be for root to be a BSON structure. An Iterator instance will always return a literal value (for strings, numbers, etc.), a BSON type (for object IDs, binary objects, etc.), or another Document or ArrayList instance depending when iterating.

Note that the introduction of the Document class makes the MongoDB\BSON\toPHP functions (and everything related to them) obsolete. I would suggest deprecating those in a separate pull request once this is merged.

Todo:

• Check memory handling (ensure we don't accidentally free a bson_t while there's still an Iterator instance around)
• Allow accessing offsets directly (public function get(string $key) in the Document class and equivalent in ArrayList)
• Define documentation requirements
• Change get() in BSON classes to throw on non-existent keys (OutOfBoundsException vs. RuntimeException)
  • ArrayList::get()
  • Document::get()
  • Create ticket for future optimisation: memoize sequential has()/get() calls
• Return type of Iterator::key(): split tests between PHP 7 (returns null) and PHP 8 (throws TypeError)
• Validate strings using bson_validate_utf8() before returning them
• Default root type map key to be array in ArrayList::toPHP()
• Remove length field in serialised output
• Refactorings
  • Use short array syntax throughout tests
  • Rename odm field to odm_ce in php_phongo_bson_state
  • Rename classname variable to type in php_phongo_bson_state_parse_type
• Expand iterator tests to comprehensively test types:
  • Nested ArrayList
  • Nested Document
  • stdClass
  • Packed arrays
  • int
  • string
  • ObjectId
• Update Document::fromBSON() tests with better explanations
• Comprehensively test invalid payload in serialisation tests for __set_state(), __unserialize(), and unserialize()

• Benchmark Iterator performance when using iterator handlers directly (instead of Iterator interface)
  • Check expectation that PHP prioritises handlers over the interface