Refactor v7_to_number -> v7_get_double and others

We wanted to avoid `to` since it implies some kind of conversion, but
these functions merely return underlying C entity.

Public API changes:

- `v7_to_number()` -> `v7_get_double()`
- `v7_to_boolean()` -> `v7_get_bool()`
- `v7_to_cstring()` -> `v7_get_cstring()`
- `v7_get_string_data()` -> `v7_get_string()`
- `v7_to_foreign()` -> `v7_get_ptr()`

Private API changes:

- `v7_to_pointer()` -> `get_ptr()`
- `v7_to_generic_object()` -> `get_generic_object_struct()`
- `v7_to_object()` -> `get_object_struct()`
- `to_js_function()` -> `get_js_function_struct()`
- `to_cfunction()` -> `get_cfunction_ptr()`

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