remove SessionVar blanket impl, replace with type impls

[a10y]

The blanket impl is a bit of a footgun, replace with impls for each of the session extension types

See discussion in #7693 (comment)

Playground link that illustrates what the blanket impl is doing: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=86edbba2a1f4d7e4fb46f281fb21a8c3

[codspeed-hq]

Merging this PR will improve performance by 11.85%

⚡ 1 improved benchmark
✅ 1129 untouched benchmarks
⏩ 33 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	275.3 ns	246.1 ns	+11.85%

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_{Comparing aduffy/session-var-imppl (9e5e0b0) with develop (269cbeb)}

Footnotes

1. 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[joseph-isaacs]

Can you add a doc str explaining that you need to do this

[gatesn]

Could we just rename the as_any/as_any_mut functions instead to not conflict?