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Add `mjs_check_arg()` which checks whether argument is provided, and
checks its type. It simplifies code and makes it smaller (because
error strings are not ad-hoc, so they are not duplicated)

As part of that, also commonize type stringifying: implement
`mjs_stringify_type` and reimplement `mjs_typeof` on top of that.

Use `mjs_check_arg()` in `mjs_string_slice()` and
`mjs_string_char_code_at()`.

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Differences between 3.1 and 3.1.1:
https://github.com/mqtt/mqtt.github.io/wiki/Differences-between-3.1.0-and-3.1.1

None of them are breaking.

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Close cesanta/dev#6974

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This can happen if the underlying LWIP TCP PCB has been destroyed (we got tcp_error_cb).

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By sending close_notify

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Frees up a bit of ram on ESP

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Flags should be 0 for all of these, except PUBREL (flags = 2).

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Message ID comes after topic, not before.

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Release pbuf sooner: after copying data from, before invoking user code

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