I find it convenient to have access to JS_ReadObject and JS_WriteObject
from JS land. That's precisely the functionality that tests/bjson.c
provides, ergo, move it into quickjs-libc.c
JSValueConst was only used for the now removed CONFIG_CHECK_JSVALUE
build mode. It is kept around as an alias for JSValue in quickjs.h to
avoid breaking everyone's source builds but remove it everywhere else.
