Fix type subsumption cache key perf regression

[majocha]

Description

Generating structural information for TType is slow. This adds some optimizations, most importantly memoization of generated type structures.
It is important to avoid holding strong references to TType in cache keys ( it was the point of #18875) so we use a weak table for memoization.

Fixes #18925

type check times are now roughly back to what they were before the regression (see #18925)

before #18875 (at commit 71167fb)
--------------------------------------------------------------------------------------------------------
|Phase name                          |Elapsed |Duration| WS(MB)|  GC0  |  GC1  |  GC2  |Handles|Threads|
|------------------------------------|--------|--------|-------|-------|-------|-------|-------|-------|
|Typecheck                           |  8.6085|  7.2778|   1574|     54|     23|      4|   1014|     83|

head (regression from 7 to 18 seconds)
--------------------------------------------------------------------------------------------------------
|Phase name                          |Elapsed |Duration| WS(MB)|  GC0  |  GC1  |  GC2  |Handles|Threads|
|------------------------------------|--------|--------|-------|-------|-------|-------|-------|-------|
|Typecheck                           | 20.3543| 18.5384|   1286|    939|    105|      3|    624|     71|


with this fix (back to circa 7 seconds)
--------------------------------------------------------------------------------------------------------
|Phase name                          |Elapsed |Duration| WS(MB)|  GC0  |  GC1  |  GC2  |Handles|Threads|
|------------------------------------|--------|--------|-------|-------|-------|-------|-------|-------|
|Typecheck                           |  8.6310|  7.3036|   1674|     54|     24|      3|    712|     71|

Caveat: this is amateur hour stuff, I have no CS background and no experience writing high performance code so any suggestions are super welcome.

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/10.0.100.md