feat: DSL parser rewrite

[AlSchlo]

Problem

The previous parser was imperative, had poorly defined syntax, was missing the entire physical side, and was not maintainable (essentially a hackathon-like POC). This PR solves all of these problems by designing a better language and using a declarative parser.

Summary of changes

• Better, more simplistic AST
• Support for ADTs
• Parsing with error recovery and useful messages
• Support map literals
• Much, much more expressive syntax to model query optimization (see test cases for now, will release a formal specification eventually)

Next steps

• Front-end: Analyzer, type checker & type-polymorphism.
• Back-end: Write a HIR prototype and interpreter, linked to the Cascades engine. (<- will focus on this first)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.61538% with 384 lines in your changes missing coverage. Please review.

Project coverage is 73.1%. Comparing base (ed593ea) to head (798aea3).

Files with missing lines	Patch %	Lines
optd-dsl/src/parser/expr.rs	82.3%	204 Missing ⚠️
optd-dsl/src/lexer/tokens.rs	0.0%	55 Missing ⚠️
optd-dsl/src/lexer/errors.rs	10.7%	25 Missing ⚠️
optd-dsl/src/parser/pattern.rs	89.3%	20 Missing ⚠️
optd-dsl/src/errors/types.rs	13.6%	19 Missing ⚠️
optd-dsl/src/parser/adt.rs	90.4%	19 Missing ⚠️
optd-dsl/src/parser/function.rs	93.9%	14 Missing ⚠️
optd-dsl/src/errors/span.rs	69.2%	12 Missing ⚠️
optd-dsl/src/parser/type.rs	94.2%	12 Missing ⚠️
optd-dsl/src/lexer/lex.rs	97.9%	4 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
optd-dsl/src/parser/module.rs	100.0% <100.0%> (ø)
optd-dsl/src/parser/utils.rs	100.0% <100.0%> (ø)
optd-dsl/src/lexer/lex.rs	97.9% <97.9%> (ø)
optd-dsl/src/errors/span.rs	69.2% <69.2%> (ø)
optd-dsl/src/parser/type.rs	94.2% <94.2%> (ø)
optd-dsl/src/parser/function.rs	93.9% <93.9%> (ø)
optd-dsl/src/errors/types.rs	13.6% <13.6%> (ø)
optd-dsl/src/parser/adt.rs	90.4% <90.4%> (ø)
optd-dsl/src/parser/pattern.rs	89.3% <89.3%> (ø)
optd-dsl/src/lexer/errors.rs	10.7% <10.7%> (ø)
... and 2 more

... and 1 file with indirect coverage changes

[skyzh]

looks like this would conflict with #26 and need to decide who merges first

[skyzh]

consider support block comments in the language?

[yliang412]

nit: Consider using raw string, i.e. `r#""# so we don't have to escape things.