Add v7 arg to _mk_ and _get_ functions

`v7_mk_undefined()` and `v7_mk_null()` are left unchanged, but marked
deprecated, and `V7_UNDEFINED` and `V7_NULL` are public now.

Xtensa code size with instrumentation: 590400 -> 590432, i.e. 32 bytes.
Xtensa code size without instrumentation: 530848 -> 531760, i.e. 912
bytes.

As objdump reveals, inside v7.c, `v7_mk_number` and others are inlined,
so the extra v7 argument is eliminated; outside of v7.c it's obviously
not inlined.

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Also, v7_get_int() is added, which currently just casts stored double
value to int

PUBLISHED_FROM=e984b7308faf2380b3de388f238e0fae0aea545d