Partially port bellard/quickjs@58f374ef42 (#214)

This commit merges JS_SetPropertyGeneric into JS_SetPropertyInternal2
and obsoletes commit b51b510 and partially obsoletes commit 8baafc4;
detachment and negative zero handling now fall out naturally.

Author: bnoordhuis

quickjs-atom.h

@@ -77,7 +77,6 @@ DEF(await, "await")
 
 /* empty string */
 DEF(empty_string, "")
-DEF(negative_zero, "-0")
 /* identifiers */
 DEF(length, "length")
 DEF(fileName, "fileName")