Env tests and fixes

[nerfZael]

Closes: #52

• Fixed an issue where a wrapper's env would override the subinvoked wrapper's env

• Fixed an issue with optional env
  It wasn't working because we were encoding an empty object which displayed an encoded size > 0, so deserialization would fail.

• Implemented a comprehensive test suite for the env feature

• This PR uses test wrappers from the following PR in the wasm test harness: Env cases refactor wrap-test-harness#64

Test cases:

• invoke method without env does not require env
• invoke method without env works with env
• invoke method with required env works with env
• invoke method with required env throws without env registered
• invoke method with optional env works with env
• invoke method with optional env works without env
• env can be registered for any uri in resolution path (this one is commented out with a TODO note and depends on an the following issue: Env not fetched when using redirected URI #79)
• subinvoke method without env does not require env
• subinvoke method without env works with env
• subinvoke method with required env works with env
• subinvoke method with required env throws without env registered
• subinvoke method with optional env works with env
• subinvoke method with optional env works without env
• subinvoker env does not override subinvoked env