feat: add rx(…, lazy=)

[camriddell]

Added an optional flag to rx(..., lazy: bool = True | False). When lazy=True chaining operations does not evaluate previous operations. Whereas when lazy=False the chaining a new operation evaluates the previous operation (original behavior).

Addresses #1105

This is the current/original behavior

fcalls = 0
def debug_call_count(value):
    nonlocal fcalls
    fcalls += 1
    return value

base = rx(input, lazy=False).rx.pipe(debug_call_count)
title = base.title() # evaluates the previous `.pipe` step
assert fcalls == 1
assert title.rx.value == 'Bob'
assert fcalls == 1 # the entire chain is not re-evaluated

When using lazy=True

fcalls = 0
def debug_call_count(value):
    nonlocal fcalls
    fcalls += 1
    return value

base = rx(input, lazy=True).rx.pipe(debug_call_count)
title = base.title() # does not evaluate the previous `.pipe` step
assert fcalls == 0
assert title.rx.value == 'Bob' # the entire chain is evaluated here
assert fcalls == 1

[camriddell]

@philippjfr all stable now. Let me know if this is the way you envisioned the API or if there's a different approach you'd like me to try out.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.26%. Comparing base (3ecffd7) to head (96d530a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1106      +/-   ##
==========================================
+ Coverage   89.22%   89.26%   +0.03%     
==========================================
  Files           9        9              
  Lines        4716     4723       +7     
==========================================
+ Hits         4208     4216       +8     
+ Misses        508      507       -1     

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[philippjfr]

@camriddell This is great, thanks for tackling this. I'll play around with it a bit but don't see any issues. @maximlt has also asked me to hold off on merging until we release a small patch release. So we should get this out some time in February I think.