ast,cgen: reduce memory usage for compiling larger v programs with long fns

[spytheman]

This PR reduces the memory usage for compiling larger V programs with long functions.

With it:

#0 18:19:11 ᛋ reduce_memory_usage_for_compiling_larger_v_programs_with_long_fns /v/vnew❱v run cmd/tools/gen_1long_fn.v 100_000 'a = b + c * d'
#0 18:19:17 ᛋ reduce_memory_usage_for_compiling_larger_v_programs_with_long_fns /v/vnew❱xtime ./vold 100000.v 
parsed 100000 statements so far from fn main.main ...
CPU: 2.77s      Real: 2.90s     Elapsed: 0:02.90        RAM: 394116KB   ./vold 100000.v
#0 18:19:23 ᛋ reduce_memory_usage_for_compiling_larger_v_programs_with_long_fns /v/vnew❱xtime ./vnew 100000.v 
parsed 100000 statements so far from fn main.main ...
CPU: 2.63s      Real: 2.73s     Elapsed: 0:02.73        RAM: 371864KB   ./vnew 100000.v
#0 18:19:28 ᛋ reduce_memory_usage_for_compiling_larger_v_programs_with_long_fns /v/vnew❱

V self compilation also benefits, although much less:

#0 18:21:54 ᛋ reduce_memory_usage_for_compiling_larger_v_programs_with_long_fns /v/vnew❱hyperfine './vprod_old -o x.c cmd/v' './vprod_new -o x.c cmd/v'
Benchmark #1: ./vprod_old -o x.c cmd/v
  Time (mean ± σ):     700.2 ms ±  10.5 ms    [User: 734.7 ms, System: 171.5 ms]
  Range (min … max):   682.4 ms … 718.1 ms    10 runs
 
Benchmark #2: ./vprod_new -o x.c cmd/v
  Time (mean ± σ):     693.2 ms ±   9.5 ms    [User: 720.7 ms, System: 168.3 ms]
  Range (min … max):   673.0 ms … 704.8 ms    10 runs
 
Summary
  './vprod_new -o x.c cmd/v' ran
    1.01 ± 0.02 times faster than './vprod_old -o x.c cmd/v'
#0 18:22:12 ᛋ reduce_memory_usage_for_compiling_larger_v_programs_with_long_fns /v/vnew❱

#0 18:23:09 ᛋ  /v/vnew❱xtime ./vprod_old -o x.c cmd/v
CPU: 0.75s      Real: 0.70s     Elapsed: 0:00.70        RAM: 304560KB   ./vprod_old -o x.c cmd/v
#0 18:23:20 ᛋ /v/vnew❱xtime ./vprod_new -o x.c cmd/v
CPU: 0.72s      Real: 0.70s     Elapsed: 0:00.70        RAM: 302556KB   ./vprod_new -o x.c cmd/v

[JalonSolov]

You wrote gen_1long_fn.v like a script, but gave it a .v extension...