Move host import mapping out of `Instance.Builder`

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:

```java

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.