* seal: Rework ext_transfer, ext_instantiate, ext_call error handling

* Deny calling plain accounts (must use transfer now)
* Return proper module error rather than ad-hoc strings
* Return the correct error codes from call,instantiate (documentation was wrong)
* Make ext_transfer fallible again to make it consistent with ext_call

* seal: Improve error messages on memory access failures

* seal: Convert contract trapped to module error

* seal: Add additional tests for transfer, call, instantiate

These tests verify that those functions return the error types
which are declared in its docs.

* Make it more pronounced that to_execution_result handles trap_reason

* Improve ReturnCode docs

* Fix whitespace issues in wat files

* Improve ReturnCode doc

* Improve ErrorOrigin doc and variant naming

* Improve docs on ExecResult and ExecError

* Encode u32 sentinel value as hex

* with_nested_context no longer accepts an Option for trie

* Fix successful typo

* Rename InvalidContractCalled to NotCallable