Add X25519 Diffie-Hellman and fe_mul121666 #127

Job	Run time
ubuntu-latest / gcc-11 / x64-baseline	50s
ubuntu-latest / gcc-12 / x64-baseline	57s
ubuntu-latest / gcc-11 / x64-avx2	1m 4s
ubuntu-latest / gcc-11 / x64-full	1m 23s
ubuntu-latest / gcc-11 / portable	1m 11s
ubuntu-latest / clang-14 / x64-full	1m 19s
macos-latest / clang / arm64	1m 13s
ubuntu-latest / gcc-12 / x64-full	1m 17s
ubuntu-latest / clang-15 / x64-full	1m 23s
ubuntu-latest / gcc-12 / portable	1m 11s
windows-latest / msvc / portable	1m 35s
ubuntu-latest / clang-15 / x64-avx2	1m 9s
macos-latest / clang / portable	1m 33s
windows-latest / msvc / x64-avx2	6m 9s
windows-latest / mingw-gcc / x64-avx2	1m 26s
ubuntu-latest / clang-14 / x64-baseline	1m 1s
macos-latest / AppleClang / arm64	48s
windows-latest / mingw-gcc / portable	1m 43s
windows-latest / mingw-gcc / x64-baseline	1m 4s
macos-latest / AppleClang / portable	1m 9s
windows-latest / mingw-gcc / x64-full	1m 46s
ubuntu-latest / clang-14 / x64-avx2	1m 10s
ubuntu-latest / gcc-12 / x64-avx2	1m 5s
windows-latest / msvc / x64-full	10m 16s
ubuntu-latest / clang-15 / portable	1m 20s
windows-latest / msvc / x64-baseline	1m 27s
ubuntu-latest / clang-15 / x64-baseline	1m 0s
ubuntu-latest / clang-14 / portable	1m 17s
	48m 46s

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