Floating point implementation refactoring

[fdxmw]

This builds on the wire_struct improvements in #480.

• Define canonical wire_struct Float types, and use them as the source of
  truth for the number of exponent and mantissa bits. See Float16 for an
  example. All operators are generic, and accept any of these Float types as
  input, and produce the same Float type as output. So if you add a Float16
  with a Float16, you get a Float16. If the operand types do not match, a
  PyrtlError is raised.

  This lets us remove _fp_wire_struct, FPTypeProperties,
  FloatingPointType, PyrtlFloatConfig because operands now carry their own
  metadata and describe themselves. For example, we can inspect the operand's
  mantissa bitwidth directly with operand.mantissa.bitwidth, rather than
  passing around another data structure.

• Remove the FloatOperations classes. They are no longer needed because the
  top-level add, sub, and mul functions are generic, and work with any
  matching Float types. The default rounding mode can be set with
  set_default_rounding_mode, and can be changed for individual operations.

• Rename the package from pyrtlfloat to just float. The full package name
  was pyrtl.rtllib.pyrtlfloat, which was redundant.

• Rename mul to mult for symmetry with signed_mult.

• Rename make_inf and friends to make_inf_like. These functions now take an
  operand that specifies the desired output bitwidths, similar to
  np.ones_like.

• Remove make_zero. It is clearer to directly set the exponent and mantissa
  to 0.

• Use dataclass to simplify some class definitions.

• Use absolute imports (https://peps.python.org/pep-0008/#imports).

• Rename some variables to improve readability.

• Update rtllib.rst to generate Sphinx documentation for the float package.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.9%. Comparing base (4ace6d3) to head (991bfd0).

Additional details and impacted files

@@              Coverage Diff              @@
##           development    #481     +/-   ##
=============================================
- Coverage         92.0%   91.9%   -0.0%     
=============================================
  Files               31      30      -1     
  Lines             7508    7437     -71     
=============================================
- Hits              6901    6834     -67     
+ Misses             607     603      -4     

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[fdxmw]

@gaborszita Here's the refactoring changes I had in mind for pyrtlfloat, please take a look when you get a chance and let me know what you think. The biggest change is that all operators now use wire_structs as their inputs and outputs, which lets us remove a bunch of data structures and code.

I also tried to clean up the documentation a bit, you can see what it currently looks like at https://pyrtl--481.org.readthedocs.build/en/481/rtllib.html#module-pyrtl.rtllib.float