rewrite parsing, evaluation, and serialization

[connorskees]

Rewrite effectively the entire compiler in order to implement all missing features. This includes

• plain css imports
• SASS/the indented syntax
• custom properties
• feature-complete @supports conditions
• @at-root conditions
• @import conditions and queries
• the builtin function keywords
• more robust support for NaN, Infinity, and -Infinity
• @forward
• media query merging
• feature complete support for complex units
• fuzzy number operations
• interpolated functions
• named arguments in varargs/splat
• a number of other minor features

This PR also implements new features that dart-sass has added through the course of this year, including

• the calculation value type and simplification. that is, something like calc(1 + 1) will now be simplified to the literal number 2
• builtin math module variables $epsilon, $max-safe-integer, $min-safe-integer, $max-number, $min-number
• builtin meta module functions meta.calc-name and meta.calc-args
• builtin list module function list.slash
• media queries level 4

Miscellaneous changes and improvements:

• remove dependency on num-rational, num-traits, num-bigint, and beef
• update spec test submodule
• bootstrap now compiles 3-4x faster on my machine

As a result of this PR, we are now failing 886/6905 spec tests. ~20% of these failures come from @use, @forward, and the module system. ~50% of these failures come from the builtin functions. It appears a large number of the current failures are either purely cosmetic (e.g. the whitespace of inline /* */ comments) or related to error messages.

Work remaining on this PR before merging:

• add back support for meta.load-css
• resolve regressions with the hwb color space
• resolve compressed output regressions
• resolve clippy and other lints
• make error spans more accurate in certain cases

Closes #52
Closes #31