* seal: Change prefix "ext_" to "seal_" for contract callable functions
The word Ext is a overloaded term in the context of substrate. It usually
is a trait which abstracts away access to external resources usually in order
to mock them away for the purpose of tests. The contract module has its own
`Ext` trait in addition the the substrate `Ext` which makes things even more
confusing.
In order to differentiate the contract callable functions more clearly from
this `Ext` concept we rename them to use the "seal_" prefix instead.
This should change no behaviour at all. This is a pure renaming commit.
* seal: Rename import module from "env" to "seal0"
* seal: Fixup integration test
* seal: Add more tests for new import module names