Fixes#1211
Previously, host functions are getting api.Module for the "originating" module,
which is the module for api.Function currently invoked, except that the api.Module
is modified by withMemory with the caller's memory instance, therefore there
haven't been no problem for most cases. The only issues were the methods
besides Memory() of api.Module, and this commit fixes them.
Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>