Change the API to heap-allocate the string and get rid of a bunch of fat stack buffers.

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Also amend api_net.js to use returned length.

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Remove the specialized URI parser, clean up code a bit.

Fix parsing of URIs with IPv6 hosts (http://[2001:2:3::4]:567/)

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`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.

----

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

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It would be probably good idea to also remove tools/docgen.py and
asciidoc.mk, but asciidoc.mk is still mentioned under `cloud/doc`, which
may contain some useful info which we'll need at least to review before
removing.

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Until I read the doc and find how to limit the retention,
otherwise it just eats all my ram and cpu and things start to fall
apart.

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