Support alloc-backed APIs in no_std

[day01]

Motivation

This PR continues the work from #400 by extending no_std support to APIs that require heap allocation.

Types such as Vec and BTreeMap are provided by Rust's alloc crate and do not inherently require std. Using alloc allows these APIs to work in no_std.

Changes

• migrate statrs-owned heap allocations from std to alloc
• make Categorical, Empirical, log_spaced, ranks, and statistical tests available in no_std
• enable matrix-backed distributions with nalgebra using its alloc and libm support
• document allocator setup for no_std applications
• add bare-metal checks for core-only, rand, nalgebra, and nalgebra,rand
• add executable Wasm verification covering representative calculations

KDE cannot be migrated because its kdtree dependency requires std.

Verification

executable no_std calculations on wasm32-unknown-unknown

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.82%. Comparing base (ad9676b) to head (4c8ca96).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #403   +/-   ##
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  Coverage   94.82%   94.82%           
=======================================
  Files          61       61           
  Lines       13539    13539           
=======================================
  Hits        12838    12838           
  Misses        701      701           

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[YeungOnion]

think moving toward libm might be nice, there wasn't much in the way of standardization of special functions, and it's a good target for shared development.

[day01]

@YeungOnion Would you mind opening a follow-up issue outlining which functions you think should move to or be shared with libm?

It would be useful to distinguish the existing overlap ;]