Systemrdl-compiler

**Bug Fixes:**
* Fix unintended behavior of a soft UDP's unassigned default value.
* If UDP is not assigned a value, rather than ignoring the assignment, assign a `NoValue` token to it. (spec 15.2.1-c)

**New features**
* `UserEnum` is now pickleable.
* Upgrade to Antlr 4.11.1 runtime
* Rework compiler's UDP preregistration API
* Deprecated `RDLCompiler.define_udp()` method in favor of new `RDLCompiler.register_udp()`
* New API allows for "soft" UDP predefinition
* Better default value & validation hooks
* See docs for more details: https://systemrdl-compiler.readthedocs.io/en/stable/api/udp.html

**Bug Fixes**
* Fix ability to define and instantiate signals inside fields. (131)

**New features**
* Add more formalized `UserEnum.define_new()` factory method for use in importers
* Add explicit `UserStruct.type_name` property.
* `UserStruct` is now pickleable.

**Bug Fixes**
* Fix missing error detection if extraneous input is passed to `RDLCompiler.expr()`
* Rework `UserEnum` to remove member name restrictions(130)
* Fix invalid importer identifier check regex.
* Fix `UserStruct` so RDL inheritance is also represented in Python class inheritance.

**New Features**
- Add numerous Node methods and properties to make working with aliases easier:
- `RegNode.is_alias`
- `RegNode.alias_primary`
- `RegNode.has_aliases`
- `RegNode.aliases`
- `FieldNode.is_alias`
- `FieldNode.alias_primary`
- `FieldNode.has_aliases`
- `FieldNode.aliases`
- `FieldNode.implements_storage` now accounts for aliases:
- Fields inside an alias register never implement storage
- A field inside a primary register may inherit additional access modes indirectly from its aliases, and therefore end up implementing storage.
- Enhance walker so that listener callbacks can control walker traversal. Listener callbacks can now optionally return a `systemrdl.WalkerAction` enumeration to direct how walker should continue.

**New Validation**
- Numerous rule checks added to check consistency of alias registers and their contents.

**Bug Fixes**
- Fix elaboration so alias registers inherit primary instance's internal/external type. 123

**New Features**
- Add `RegNode.is_interrupt_reg`
- Add `RegNode.is_halt_reg`
- Include offending text in "illegal name" import exceptions
- `FieldNode.implements_storage` now returns True for counters, interrupts, and sticky/stickybit fields

**New Validation**
- Enforce that `next` property requires field to be hw writable.
- Add check for conflicting edge-sensitive interrupt that uses whole-field sticky property.
- Add check for conflicting `sticky`/`stickybit` with `we`/`wel` properties
- A field that uses `sticky`/`stickybit` properties shall be hw writable
- Illegal to use `enable`/`mask`/`haltenable`/`haltmask` on non-interrupt fields
- Value of `incrwidth`/`decrwidth` shall be between 1 and the width of the counter
- Use of `overflow`/`underflow` is pointless on counters that saturate. Emit error.

**Bug Fixes**
- Fix `intr type` to return None for non-interrupt fields
- Fix uncaught error if local property assignment is attempted in the root namespace

**New Features**
- Upgrade to Antlr 4.9.3
- Add `FieldNode.is_up_counter` and `FieldNode.is_down_counter`
- Add `PropertyReference.width`
- Add `MemNode.is_sw_readable` and `MemNode.is_sw_writable` (115)
- Add `skip_top` option to `RDLWalker.walk()` method

**New Validation**
- Improve validation of counter properties
- Add validation of property reference widths
- Require that signals used in resets shall have activehigh/activelow specified
- Flag illegal rclr/rset/woclr/woset properties if field is not readable/writable

**Bug Fixes**
- Fix default return value for incrvalue/decrvalue
- Fix incorrect validation of incrvalue/decrvalue reference widths
- Fix incorrect spelling of decrsaturate/decrthreshold RHS property references (was previously decsaturate/decthreshold!)