frozen-abi: add imbl::GenericHashMap

[2501babe]

anza-xyz/agave#10762 intends to replace im with imbl as the backing store of StakesCache. the current crate is no longer maintained, while the new crate is a drop-in replacement under active development, with significant improvements

testing with my own benchmarks (anza-xyz/agave#10760) and confirmed by alessandro on mainnet, this change speeds up epoch cutover by 30-50%. however, we must implement AbiExample for imbl::HashMap to satify the frozen-abi contract on Bank. the way this is done for im is a simple impl in sdk

one of the challenges with copying this for imbl is the perf improvements we want are in imbl@v7.0.0, but this version depends on rust 1.85, while sdk enforces a minimum supported version of 1.81. the alternative is to put imbl::HashMap under a newtype wrapper and implement AbiExample in agave, with no sdk changes. this ends up being quite unpleasant because it requires us to write a different Serialize for the frozen-abi feature gate that returns some other type, because a wrapper over imbl::HashMap does not save us from needing AbiExample on every underlying type. aside from being ugly, i feel this violates the spirit of frozen-abi because we are explicitly lying to it about the serde representation of our type

"if only there were a better way!" there is. but i leave it to you to decide if i am a clever sort or if im guilty of Crate Crimes. this pr does the simple AbiExample for imbl::HashMap in sdk, identical to the treatment of im::HashMap. but i allow both imbl@v6 and imbl@7 in Cargo.toml, and give sdk a Cargo.lock enforcing 6. this means sdk is happy on rust 1.81. but the open version allows agave to use 7 without issue, getting us the perf we desire

none of this should affect the actual thing these tests care about, which is the snapshot format. im::HashMap and imbl::HashMap both go to the serde serialized representation of a platonic hashmap

[2501babe]

imbl::HashMap is just a type alias for this struct with two of the generic params held fixed

[joncinque]

Hehe very clever! Confirming the types are equivalent in v6 and v7, let's just add a comment so we don't mess this up in the future

[joncinque]

Can you just add a comment here so we don't mess this up in the future? Something like

# Use v6 in the SDK to maintain a lower rust-version, but allow v7 for Agave which uses a higher rust-version and needs v7 performance

[2501babe]

73b053c i put it to the right rather than above to copy the style of the serde comments

[joncinque]

Looks great, thanks!