Typechecking doesn't enforce communication immutability requirements

[eholk]

It looks like we'll still attempt to compile attempts to send mutable data on channels or in spawn arguments. This should be disallowed.

[eholk]

It looks like this depends on having the kind system in place (#234 and #132).

[brson]

The only thing left to do here is immutable closures (fn~) or bare functions

[brson]

Depends on #1022

[brson]

Done.