Disabling the package item injection routine used in the enum conversion
when there were no items to inject exposed cases where conversions would
generate duplicate declarations. The hierarchical constant and param
type conversions were trivially affected. The package conversion could
inject class items within a generate region and then re-inject them
outside of that generate region. The package conversions now uses an
upgraded generate region flattening utility to ensure injected class
items are seen. This also includes coverage for a conflict which
occurred even without the enum conversion change.

- remove needless comments generated during interface conversion
- filter null statements even in verbose mode
- hoist trace comment statements if doing so would prevent the need to
  wrap a task or function body in a block
- add traces for global declarations
- add traces for parameter declaration lists
- add traces for module items within generate blocks
- avoid generating duplicate traces before declarations

- fix int type conversion not visiting function return type expressions
- add node-based traversal for decls visiting top level types and exprs
- explicit expr and type recursion in package ident resolution
- expose mapBothM traversal helper
- avoid double-visiting of decl types in certain conversions

- avoid packing memories used with read mem tasks
- fix detection of $readmem[hb] in logic conversion

- standardize number casting
- fix extension of casts of unsized literals to 32 bits

- general refactoring in decl parsing
- restrict charge strength to trireg
- require const vars to be initialized
- forbid const net declarations
- disallow run-on declarations in packages and classes

In places where an optional space was previously allowed, allow any
number of arbitrary whitespace characters, in line with the spec and
other tools.

Data.Hashable can produce hashes differing little in their upper bits if
only the last character of the string is changed. Because we were using
the upper bits of the hash in shortHash, this could lead to avoidable
hash collisions. This change includes minor simplification optimizations
which surfaced this collision in the existing struct_ident_scope test.

With permission from Tom Hawkins, I have updated sv2v's existing BSD
3-Clause license to match the exact text of the standard BSD 3-Clause
license. The wording changes, listed below in full, are entirely
innocuous. The copyright attribution is now simpler. The license heading
and link have been removed from the readme. These changes enable the
license to be appropriately detected by GitHub's license detection tool,
hopefully eliminating any potential confusion about the license in use.

- "author" ==> "copyright holder"
- "his" ==> "its"
- "AUTHORS" ==> "COPYRIGHT HOLDERS AND CONTRIBUTORS"
- "AUTHORS" ==> "COPYRIGHT HOLDER"

- remove outdated flexible partial type eq and show instances
- properly disallow incomplete declarations
- disallow var after net type
- remove unused edge case in InterfaceT representation

- support for additional assignment statements in loop initializations
- greatly improved error messaging in these contexts
- decl parser takes in the ending token; significant related refactoring
- pass through elaboration system tasks
- removed non-blocking assignment operator precedence hack
- preliminary nosim test suite for features unsupported by iverilog

- all decl tokens are given an accurate starting position
- key grammar productions return token positions to facilitate the above
- helpers for standardized parse error generation
- replaced annoying pattern-matching type argument restrictions
- moving away from dumping raw decl tokens in error messages

- new net decl to replace net pseudo-type
- support nets with complex base types, including typenames
- support var declaration type prefix for all data types
- support var as lone type shorthand
- refactor AST representation of strengths
- traversal helpers for treating nets as variables
- use decl traversals where appropriate

- automatic pass-through test suite coverage
- fix codegen for interface based typedefs
- allow declaration of generic interface ports in module body
- fix codegen for comments within for loop initialization
- fix codegen of class param decls