Internals: Make the constant pool a regular package (#8512)

The constant pool is now an ordinary package, created with the netlist
and instantiated under $root like any other package, and most special
handling has been removed, including AstConstPool. Lookups go through
V3ConstPool, which has a singleton instance owned by V3Global. Static
methods on V3Common form the public interfce to add/find constant pool
entries.

With that, the constant pool is usable at any stage during compilation,
so enum and dimension tables created by V3Width, and enum value tables
created by V3Randomize now also live in the constant pool instead of
$unit, so identical tables are shared.

Associative array constants are handled separately from unpacked
tables, which used to be broken but unused.

Emitted constant pool variables use direct initialization, and
`constinit` with C++20 where the type allows it. This ensures we don't
change a run-time in a way that would result in unintended code size
increase.

-fno-merge-const-pool, which was introduced years ago but never prompted
a bug report is deprecated and has no effect.

This is also prep for future work.

docs/guide/exe_verilator.rst

@@ -852,6 +852,11 @@ Summary:
 
 .. option:: -fno-merge-const-pool
 
+   Deprecated and has no effect (ignored).
+
+   In versions before 5.054: Disable merging constant pool entries that have
+   the same value but different data types.
+
 .. option:: -fno-reloop
 
 .. option:: -fno-reorder