seal: Change prefix and module name from "ext_" to "seal_" for contract callable functions (#6798)

* 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

substrate/frame/contracts/fixtures/self_destructing_constructor.wat

@@ -1,5 +1,5 @@
 (module
-	(import "env" "ext_terminate" (func $ext_terminate (param i32 i32)))
+	(import "seal0" "seal_terminate" (func $seal_terminate (param i32 i32)))
 	(import "env" "memory" (memory 1 1))
 
 	(func $assert (param i32)
@@ -13,7 +13,7 @@
 
 	(func (export "deploy")
 		;; Self-destruct by sending full balance to the 0 address.
-		(call $ext_terminate
+		(call $seal_terminate
 			(i32.const 0)	;; Pointer to destination address
 			(i32.const 8)	;; Length of destination address
 		)