feat(data-structures/unstable): add MultiMap

[tomas-zijdemans]

Adds MultiMap<K, V> under @std/data-structures/unstable. It's a Map-like container where each key holds an ordered list of values.

Covers the common "values keyed by group" case (e.g. parsed query strings, HTTP headers, users grouped by role) without hand-rolling Map<K, V[]> wrappers at every call site.

I plan to use it in the cache and xml module.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (3e91d1e) to head (1b535e6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7100      +/-   ##
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+ Coverage   94.51%   94.53%   +0.02%     
==========================================
  Files         630      631       +1     
  Lines       50970    51165     +195     
  Branches     9090     9127      +37     
==========================================
+ Hits        48175    48371     +196     
  Misses       2224     2224              
+ Partials      571      570       -1     

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[bartlomieju]

Nice work — well-documented, well-tested, and covers a real gap. A few things to address:

valueCount getter

size counting distinct keys follows Map convention, but multimap users very commonly need total value count. Consider adding a valueCount getter, cheaply maintained with an internal counter (increment in add, decrement in deleteEntry, reset in clear/delete). Without it the only way is to iterate everything.

Deno.customInspect

Not a blocker (other unstable data structures don't have it either), but [Symbol.for("Deno.customInspect")] would improve DX — console.log(multimap) would show something useful rather than an opaque object.

[tomas-zijdemans]

Thanks for the thorough review.

• valueCount: Added a valueCount getter, backed by an internal counter as suggested
• Deno.customInspect: Added one small hook so console.log(map) prints nicely. Opted not to add a Node-side twin (nodejs.util.inspect.custom), keeping this in line with the other data_structures modules. The Deno-side hook alone already fixes the unreadable default output; we can add the Node twin later if there's demand.
• get() complexity note: Clarified the table row to Linear in the bucket size (copy) as suggested
• SameValueZero consistency: Added a short comment on hasEntry() noting that .includes() already matches the SameValueZero loop in deleteEntry()
• Simplify forEach(): Removed the thisArg-undefined branch; one loop with callbackfn.call(thisArg as T, ...) covers both cases.
• Iterator style: Benchmarked hand-rolled vs. generator (1.7×–4.4× faster, never slower). Rewrote groups() as a hand-rolled iterator to match entries()/values(); kept keys() delegating to the native Map iterator.

[bartlomieju]

Thanks!