Improve performance of join/1 on large arrays (#3639)

join/1 folded each element as `(acc + $x) + elem`, with the element
branch ending in `.//""`. Both leave a second live reference to the
accumulator: the nested + feeds the same `.` to both operands, and the
`//` leaves a forkpoint below it on the stack. `jvp_string_append`
therefore cannot grow the accumulator in place, and copies the whole
string once per element.

Pipe `acc + $x` into `. + elem` and drop the `//`, since adding null to
a string is already a no-op. join/1 is now linear: joining 8000
strings of 1000 chars goes from 6.5s to 0.06s, with output and error
messages unchanged.

tests/jq.test

@@ -2076,6 +2076,14 @@ try join(",") catch .
 ["1","2",[3,4,5]]
 "string (\"1,2,\") and array ([3,4,5]) cannot be added"
 
+join("-")
+{"a":"x","b":1,"c":null,"d":false}
+"x-1--false"
+
+try join(1) catch .
+["a","b"]
+"string (\"a\") and number (1) cannot be added"
+
 {if:0,and:1,or:2,then:3,else:4,elif:5,end:6,as:7,def:8,reduce:9,foreach:10,try:11,catch:12,label:13,import:14,include:15,module:16}
 null
 {"if":0,"and":1,"or":2,"then":3,"else":4,"elif":5,"end":6,"as":7,"def":8,"reduce":9,"foreach":10,"try":11,"catch":12,"label":13,"import":14,"include":15,"module":16}