Add support for (ref ...) / (ref null ...) constructs

[zherczeg]

This large patch adds function references support for those parts of the wabt library, which already present in the code. The current code is designed for an older (outdated) variant of the function references proposal.

Key changes:

• Var variables can optionally store type data. This way there is no need for a Var/Type field pair in many cases.
• CallRef now needs to explicitly provide its reference type (no auto detection)
• Tracking local reference initialization (non nullable refs must be set before using them)
• Supporting type equality comparisons in the validator and in the interpreter
• Adds a code for reading/writing 33 bit integers (leb format)
• Remove EndTypeSection in shared validator, types can be validated earlier
• Improve named reference resolving in the text parser