IL: fix the per-reader string cache sizing

[auduchinok]

ILMetadataReader keeps two string tables per referenced assembly. Both were sized from something
unrelated to how many strings are actually read.

cacheStringHeap was sized stringsStreamSize / 50 + 1, i.e. from the length of the #Strings stream.
Only a small fraction of a #Strings heap is ever read, so the table sat around 11% full — one
nearly-empty table per reference. It is now sized to grow.

memoizeString interned one computed string: the ns + "." + name concatenation in
readBlobHeapAsTypeName. Every caller of that function is already cached or one-shot per row
(typeDefReader, seekReadTypeDefAsTypeRefUncached, seekReadTypeRefUncached, and the exported-type
readers), so the table could only pay when two different rows produce identical text. It went through
Tables.memoize, whose fixed 1000-entry capacity cost about 3.0 MB in buckets across a large reference
set while collapsing at most 0.24 MB of duplicate names — so the table is removed rather than resized.
Tables.memoize keeps its ilmorph.fs caller.

Retained memory after ParseAndCheckProject, mean of 3 runs in fresh processes per project:

Project	Before	After	Retained
consoleapp	33.25 MB	31.11 MB	-2.14 MB (-6.4%)
Oxpecker	69.65 MB	65.77 MB	-3.88 MB (-5.6%)
IcedTasks	47.10 MB	44.93 MB	-2.17 MB (-4.6%)
FsToolkit	69.97 MB	67.86 MB	-2.11 MB (-3.0%)
Fantomas.Benchmarks	64.26 MB	61.33 MB	-2.93 MB (-4.6%)
Prime	100.99 MB	99.44 MB	-1.55 MB (-1.5%)
Fantomas.Core	107.74 MB	106.21 MB	-1.53 MB (-1.4%)
Fantomas.Core.Tests	140.20 MB	137.61 MB	-2.59 MB (-1.8%)
FSharp.Common	261.81 MB	251.68 MB	-10.13 MB (-3.9%)
fcs	1226.73 MB	1223.99 MB	-2.74 MB (-0.2%)

The saving is two tables per reader, so it scales with the number of referenced assemblies rather than
project size: the smallest subject gains most in relative terms, and FSharp.Common, with 489 references,
most in absolute terms. Total allocation drops 1-34 MB per project. Analysis time is unchanged within
measurement noise.

Without the intern table, ~2,077 duplicate name strings survive per FSharp.Common analysis (~0.24 MB,
an upper bound). 98% are one structural pair: the same type def read once as ILTypeDef.Name and once
as ILTypeRef.Name. That is the 0.24 MB the table used to collapse, and it is well below what the table
itself cost.

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Change path	Release notes path	Description
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