crypto/elliptic: deprecate big.Int interface

[FiloSottile]

Exposing big.Ints in crypto/elliptic was a mistake (#52075, #50974, #37294), and the package is slow and not constant time.

I've been running a large remediation project for a few releases to replace the guts of the elliptic curve implementations.

This issue tracks related efforts for Go 1.19. Some related issues:

• crypto/elliptic: normalize behavior of invalid curve points #50975
• crypto/elliptic: automatically upgrade CurveParams for known curves and deprecate custom ones #34648
• crypto: import fiat-crypto implementations #40171
• crypto/ecdh: new package #52221

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Change https://go.dev/cl/382994 mentions this issue: crypto/elliptic: inline marshaling into nistec pointFromAffine

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Change https://go.dev/cl/390514 mentions this issue: crypto/elliptic: don't use big.Int.Mod/Mul in generic P-256

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Change https://go.dev/cl/315273 mentions this issue: crypto/elliptic: refactor package structure

[gopherbot]

Change https://go.dev/cl/396255 mentions this issue: crypto/elliptic: clean up and document P-256 assembly interface

[gopherbot]

Change https://go.dev/cl/390754 mentions this issue: crypto/elliptic: split up P-256 field and group ops

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Change https://go.dev/cl/395775 mentions this issue: crypto/elliptic: move P-256 amd64/arm64 assembly to nistec

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Change https://go.dev/cl/382995 mentions this issue: crypto/elliptic: panic when operating on invalid points

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Change https://go.dev/cl/396414 mentions this issue: crypto/elliptic: generate nistec group implementations from template

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Change https://go.dev/cl/396935 mentions this issue: crypto/elliptic: implement UnmarshalCompressed in nistec

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Change https://go.dev/cl/390134 mentions this issue: crypto/elliptic: move generic P-256 implementation into nistec

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Change https://go.dev/cl/360015 mentions this issue: crypto/elliptic: use generics for nistec-based curves

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Change https://go.dev/cl/398674 mentions this issue: crypto/elliptic/internal/nistec: move to crypto/internal/nistec