Remove frozen-abi build script

[kevinheavey]

Problem

Since all frozen-abi stuff is feature gated these days, we don't need #[cfg(RUSTC_WITH_SPECIALIZATION)] any more. We can just rely on the frozen-abi feature that already exists for all the crates that need it.

Summary of Changes

• Replace all checks for RUSTC_WITH_SPECIALIZATION with checks for feature = "frozen-abi"
• Remove all frozen-abi build scripts (these only serve to set RUSTC_WITH_SPECIALIZATION). In a few cases, there is still a build script doing other tasks, and I've only removed the frozen-abi parts.
• Remove all rustc_version dependencies, as these are only used by frozen-abi build scripts
• Make the two frozen-abi crates do almost nothing by default unless the frozen-abi feature of those crates is activated. The point of this is to make sure the workspace compiles on stable Rust - the frozen-abi feature is only for nightly Rust

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.

[ryoqun]

I appreciate these changes and think this is in good direction.

just curious. how much is build time reduced?

[ryoqun]

also, could you fix conflicts? (sorry for the delayed code-review btw...)

[kevinheavey]

just curious. how much is build time reduced?

Seems to be 10-100 ms when the frozen-abi feature is deactivated. That's more than I would have guessed since the build script is empty when the feature is deactivated.

[ryoqun]

(it seems that "rustc_version 0.4.1" itself can't still get rid of because some trnasitive deps (namely ark-ff, cast and curve25519-dalek) depends on that..)

[ryoqun]

seems this isn't needed to begin with.

[ryoqun]

lgtm; come to think of it, feature-based gating makes sense then custom build.rs and custom cfg... really thanks for all the efforts of fixing this one by one.