go/types/objectpath: reduce allocations with sync.Pool

[ewhauser]

Use sync.Pool to reuse byte buffers and finder structs,
reducing allocations by ~52% in heavy-usage scenarios.

• Add pathBufPool for reusing path encoding buffers
• Add smallInts lookup table for integers 0-99
• Add finderPool for reusing finder structs

Benchmarking on a 9000-package monorepo with ~40 analyzers showed:

• Allocations: 929MB → 445MB (52% reduction)
• GC cycles: 21 → 14 (33% reduction)
• GC pause time: 1.77ms → 0.81ms (54% reduction)

Fixes golang/go#77059

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Message from Daniel Morsing:

Patch Set 1:

(2 comments)

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Patch Set 1:

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