py: fix native emitter to work with async with statements

[dpgeorge]

This fixes the native emitter so that async with statements now work with it.

---

This PR also includes a new "debug native emitter" that can be used to print out pseudo instructions generated by the native emitter, to help debug issues with it. Eg with the following test.py file:

@micropython.native
def f():
    return 1

mpy-cross can be used as follows to get debug output from the native emitter:

$ ./build/mpy-cross -march=debug test.py
ENTRY(num_locals=6)
    load(r_fun_table, r_arg1, 1)
    load(r_local3, r_fun_table, 2)
    load(r_fun_table, r_fun_table, 3)
    load(r_fun_table, r_fun_table, 0)
    mov_local_reg(local_0, r_arg1)
    mov_reg_imm(r_arg1, 1=0x1)
    mov_local_reg(local_3, r_arg1)
    mov_reg_local_addr(r_arg1, local_0)
    call_ind(setup_code_state)
    mov_reg_imm(r_ret, 3=0x3)
    jump(label_0)
    dead_code load(r_ret, r_fun_table, 0)
    dead_code jump(label_0)
label(label_0)
EXIT(0)

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:  +584 +0.071% standard
      stm32:  +232 +0.059% PYBV10
     mimxrt:  +240 +0.066% TEENSY40
        rp2:  +232 +0.026% RPI_PICO_W
       samd:  +240 +0.090% ADAFRUIT_ITSYBITSY_M4_EXPRESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.42%. Comparing base (c1882e4) to head (758f771).

❗ Current head 758f771 differs from pull request most recent head 0619f26

Please upload reports for the commit 0619f26 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #15310   +/-   ##
=======================================
  Coverage   98.42%   98.42%           
=======================================
  Files         161      161           
  Lines       21204    21234   +30     
=======================================
+ Hits        20870    20900   +30     
  Misses        334      334           

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