[REVIEW] Add explicit conversion for fixed_point to bool.

[bdice]

This PR adds an explicit conversion template specialization from fixed_point values to bool. Below, I explain the problem this is solving in case it's helpful for someone in the future.

While developing the AST interpreter (#5494), I implemented a logical "and" operator. I noticed that the expression fixed_point_value && true tries to convert fixed_point to a bool with this cast operator but fails to compile (bool passes the is_integral trait, and is thus an allowed cast by the is_supported_construction_value trait).

The compilation failure is in left_shift (it can't compile this line, which tries to multiply the cast-to type bool by the scale factor. You might wonder why this is happening -- the conversion is marked explicit. The cause is that logical operators perform explicit conversions ^{[1, 2]}. References for this behavior are below.

^[1] https://en.cppreference.com/w/cpp/language/operator_logical (bold added)

If the operand is not bool, it is converted to bool using contextual conversion to bool: it is only well-formed if the declaration bool t(arg) is well-formed, for some invented temporary t.

^[2] https://en.cppreference.com/w/cpp/language/implicit_conversion (under "Contextual conversions", bold added)

In the following contexts, the type bool is expected and the implicit conversion is performed if the declaration bool t(e); is well-formed (that is, an explicit conversion function such as explicit T::operator bool() const; is considered). Such expression e is said to be contextually converted to bool.

• the controlling expression of if, while, for;
• the operands of the built-in logical operators !, && and ||;
• the first operand of the conditional operator ?:;
• the predicate in a static_assert declaration;
• the expression in a noexcept specifier;

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codereport]

Looks great! Thanks for the fix! Only small change to Doxygen comment

[codecov]

Codecov Report

Merging #5859 into branch-0.15 will increase coverage by 0.32%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.15    #5859      +/-   ##
===============================================
+ Coverage        84.10%   84.43%   +0.32%     
===============================================
  Files               80       80              
  Lines            13094    13403     +309     
===============================================
+ Hits             11013    11317     +304     
- Misses            2081     2086       +5     

Impacted Files	Coverage Δ
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/custreamz/custreamz/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/applyutils.py	98.75% <0.00%> (+0.02%)	⬆️
python/cudf/cudf/core/join/join.py	92.41% <0.00%> (+0.03%)	⬆️
...ython/custreamz/custreamz/tests/test_dataframes.py	98.80% <0.00%> (+0.03%)	⬆️
... and 29 more

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