builtin: zero out internal map/array pointers on m.free(), to reduce the work for the GC mark phase for non escaping maps/arrays, used in hot loops

[spytheman]

Note the maximum RAM usage for the following program. It is significantly lower with this PR (1611608KB vs 1918948KB):

Program:

fn work_with_map() {
	mut m := map[string]int{}
	for i in 0 .. 4_000_000 {
		m['k_${i}'] = i
	}
	eprintln('   > m.len: ${m.len:10}')
	unsafe { m.free() }
}

fn main() {
	eprintln('start')
	for k in 0 .. 10 {
		work_with_map()
		eprintln('k: ${k:10}')
	}
	eprintln('stop')
}

On master:

#0 17:19:41 ᛋ master /v/vnew❱xtime ./m_old
start
   > m.len:    4000000
k:          0
GC Warning: Repeated allocation of very large block (appr. size 67112960):
        May lead to memory leak and poor performance
   > m.len:    4000000
k:          1
   > m.len:    4000000
k:          2
   > m.len:    4000000
k:          3
   > m.len:    4000000
k:          4
GC Warning: Repeated allocation of very large block (appr. size 17362944):
        May lead to memory leak and poor performance
   > m.len:    4000000
k:          5
   > m.len:    4000000
k:          6
   > m.len:    4000000
k:          7
   > m.len:    4000000
k:          8
GC Warning: Repeated allocation of very large block (appr. size 63430656):
        May lead to memory leak and poor performance
   > m.len:    4000000
k:          9
stop
CPU: 41.41s     Real: 39.06s    Elapsed: 0:39.06        RAM: 1918948KB  ./m_old
#0 17:20:27 ᛋ master /v/vnew❱

With this PR:

#0 17:20:33 ᛋ builtin_make_gc_mark_more_efficient_for_freed_v_maps_and_arrays /v/vnew❱xtime ./m_new
start
GC Warning: Repeated allocation of very large block (appr. size 39600128):
        May lead to memory leak and poor performance
GC Warning: Repeated allocation of very large block (appr. size 63430656):
        May lead to memory leak and poor performance
   > m.len:    4000000
k:          0
   > m.len:    4000000
k:          1
   > m.len:    4000000
k:          2
GC Warning: Repeated allocation of very large block (appr. size 71360512):
        May lead to memory leak and poor performance
   > m.len:    4000000
k:          3
   > m.len:    4000000
k:          4
   > m.len:    4000000
k:          5
   > m.len:    4000000
k:          6
   > m.len:    4000000
k:          7
   > m.len:    4000000
k:          8
   > m.len:    4000000
k:          9
stop
CPU: 42.96s     Real: 39.42s    Elapsed: 0:39.42        RAM: 1611608KB  ./m_new
#0 17:21:18 ᛋ builtin_make_gc_mark_more_efficient_for_freed_v_maps_and_arrays /v/vnew❱

[medvednikov]

All tests pass. Good job!