Isolate the `_decimal` extension module

[CharlieZhao95]

This issue is used to track the PRs split from gh-103092. Isolate the _decimal extension module by:

• Establish a global module state and convert static types to heap types
• Move other global static variables to the global module state(possibly multiple PRs)
  • DecimalException
  • basic_context_template
  • tls_context_key
  • cached_context
  • current_context_var
  • default_context_template
  • extended_context_template
  • round_map
  • Rational
  • SignalTuple
  • External C-API functions
• Convert the global module state to true module state
  • signal_map
  • cond_map
  • convert global state to module state

See original issue Isolate Stdlib Extension Modules and PEP-687 for details.

Linked PRs

• gh-106078: Convert _decimal types to heap types #106079
• gh-106078: Move DecimalException to global state #106301
• gh-106078: Move context template to decimal module global state #106346
• gh-106078: Move static objects related to CONTEXTVAR to the decimal module global state #106395
• gh-106078: Move static variables initialized once to decimal module global state #106475
• gh-106078: Move external C-API functions to decimal module global state #106616
• gh-106078: Prepare to isolate decimal module #106880
• gh-106078: Isolate decimal module #107287
• gh-106078: Suppress the warning caused by multi-phase initialization of decimal #107524

[erlend-aasland]

What is your next step, @CharlieZhao95?

[CharlieZhao95]

What is your next step, @CharlieZhao95?

I think next we can move the remaining static global PyObject to global_state:

• DecimalException
• basic_context_template
• current_context_var
• default_context_template
• extended_context_template
• round_map
• Rational
• SignalTuple
• External C-API functions

This shouldn't be too complicated. The only question is whether we should move these external-API functions, which simply hold function pointers used by numeric types, such as nb_multiply, nb_floor_divide and so on. They will not change during the lifecycle of the program.

[erlend-aasland]

Pick the low-hanging fruit first. It is better with multiple small PRs than one big.

[CharlieZhao95]

Pick the low-hanging fruit first. It is better with multiple small PRs than one big.

Okay! let's go step by step. I will put the objects listed above into multiple small PRs.