[Support] Add FVInt, a four-valued arbitrary precision integer

Add the `FVInt` class to CIRCT's support library. This class can
represent arbitrary precision integers where each bit can be one of the
four values 0, 1, X, and Z. The name intends to suggest a *four-valued
APInt*, with the option to also introduce a *nine-valued* `NVInt` in the
future.

Internally, `FVInt` uses two `APInt`s to store its data: `value` stores
whether a bit is 0/X or 1/Z, and `unknown` stores whether a bit is known
(0 or 1) or unknown (X or Z). Together they allocate 2 bits of storage
for each of the `FVInt`'s digits, which allows for four different values
per digit. This representation as `value` and `unknown` makes many of
the logical and arithmetic operations pretty straightforward to
implement. Most four-valued operations can be trivially implemented by
performing the equivalent two-valued operation on `value`, and then
accounting for X and Z bits through a few logic operations on `unknown`.

Note that Slang defines its own version of this (`SVInt`). But since
Slang is an optional dependency of CIRCT, it makes sense to have a CIRCT
equivalent that is built around LLVM's `APInt`, for use in our dialects.

This first version of `FVInt` has a rather incomplete set of operations,
but it covers basic AND, OR, XOR, NOT, negation, addition, subtraction,
and multiplication as a proof-of-concept. The remaining operations will
be added in future commits. We are also going to need a four-valued
equivalent of `IntegerAttr` based on `FVInt`.

This commit is motivated by the Slang frontend, which now supports
enough of SystemVerilog to make some test designs start to hit the lack
of number literals with X and Z.