Paxter

- Once again, the entire Paxter language grammar was re-designed.
_No longer compatible with all previous versions._
- The identifier section of function-call -expressions (called a “command”)
and the phrase expression were unified into the same pattern.
Now, it is possible to evaluate a phrase expression into a function
and use it in the function-call -expression.

- The entire Paxter language grammar was re-designed from the ground up.
_No longer compatible with all previous versions._
- Parsed tree transformers are now called renderers.
- Simple Snake flavor of tree transformers (i.e. renderers)
were removed and no longer exists.
- ReadTheDocs documentation is now set up.

- Changes in Paxter language Specification
- `PaxterMacro` now allows options just like `PaxterFunc`
- For each key-value pair in an option list,
the key part now becomes optional in place of the value part.
Therefore, the option list of the form `[1,"2",v3,k4=4,k5="5",k6=v6]`
is a valid Paxter language syntax
(formerly, the first two key-value pairs were unacceptable).
- New utility decorators in Simple Snake:
`with_env`, `with_node`, and `DefinitionSet`.
- Way better unit test coverages and docstrings

- Optimized parser (remove redundant steps, refactor code, etc.)
- Package reorganization (e.g. flavored tree transformers now in `paxter.flavors`)
- **Simple Snake** behavior changes:
- A line ending with backslash will remove the backslash itself
along with the newline character next to it
- Introduce `load!` macro to load pre-defined functions
- Introduce `if` and `for` special functions
- Better unit test coverages and docstrings

- Remove debugging print statement from transformer

- Fixed missing quotation mark rule in JSON string literal lexer
- Introduced simple python-flavored parsed tree transformer