builtin/item.lua: callbacks with copies of positions and nodes rather than recycle the same ones, which callbacks can modify

It'd take years to figure out what caused the accident.

wctomb(NULL, _) returns "nonzero if the encoding has nontrivial shift state, or zero if the encoding is stateless."

I assume the intentation was to get the size of the target buffer.
Use MB_CUR_MAX for this.

As mbtowc(_, _, 1) reads at most one char, everything other than a
return value of 1 is an error. Since the input strings are static,
an assert protects against future changes.

Likewise, wctomb should currently never encounter a character, which
actually needs a multibyte representation.