Move host import mapping out of Instance.Builder

[evacchi]

the build() method of the Instance.Builder() includes a lot of logic to map declared imports to the provided imported values (in particular, see mapHostImports(). This causes some idiosyncrasies such as having a "strange" signature for the main Instance constructor:

    public Instance(
            Module module,
            Global[] globalInitializers,
            int importedGlobalsOffset,      // these offsets should not be really handled at this point
            int importedFunctionsOffset,
            int importedTablesOffset,
            Memory memory,

the mapping between imports of a Module and the provided ImportValues could be handled externally; in particular, in the light of having an externalized "Linking" mechanism, the Linker, might provide a strategy to resolve such mappings.

In the current state, even without a Linker it might be even possible that Store (or a related, new class) could be a better place to handle such a logic.