Commonize arg checking in cfunctions

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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