Improve rex env binding

[davidlatwe]

This PR address following two issues.

Can't check if env contains var with keyword in

In package.py, this won't work

# package.py
def commands():
    if "FOO" in env:
        print("A")
    else:
        print("B")  # cannot be reached

Because "FOO" in env or "FOO" in env.keys() always return True, no matter the env actually contains the var or not.

Commit ffbbfbc fixed this.

Can't use keyword or to get default value

Sometimes I'd like to set default value like this

# package.py
def command():
    foo = str(env.get("FOO") or "hello, foo.")

But if key "FOO" not in env, RexUndefinedVariableError is raised.

In regular dict, the get() method can take default value if the key not exists. However, the EnvironmentDict.__getitem__ always return an EnvironmentVariable instance, so the default value never had a chance. So the alternative is using or. But if key not exists, error raised when it's being evaluated in EnvironmentVariable.__bool__() method.

I think __bool__ should never raise error, hence 658322f.