Support VariableList longer than 2**31 on 32-bit architectures

[color-typea]

• Improves handling large VariableList for 32-bit architectures
  • loudly crash now vs. silently overflow (and produce wrong results) previously
• Adds feature to enable capping typenum to usize conversion to usize::MAX
• Tests + github actions

Why:

• BeaconState.validators is defined as VariableList<Validator, typenum::U1099511627776> (2**40)
• U1099511627776::to_usize() returns 0 on 32-bit architectures
• ... such as riscv32 or wasm32 that are popular choices for Zero-Knowledge VMs (such as SP1)

How:

typenum_helpers.rs:

• Conditional compilation on 64-bit vs. 32-bit architecture
• By default, loudly panic if overflow happens on 32-bit
• Added a feature to allow capping to usize::MAX instead - allows working with "virtually larger than usize" VariableLists that are guaranteed to have less than usize::MAX elements (i.e. only larger in typenum)

tree_hash.rs

• Split on the pointer size
• no changes for 64-bit arches - only wrapped into the module for more convenient access.
• For 32-bit, cap the actual tree computation to 2**29, than "expand" (O(depth) time) to the target tree depth by hashing with zerohash of corresponding level.
  • Should've been 2**31, but there are some downstream operations in ethereum_hashing/tree_hash that cause overflows with larger values - so this is empirically determined maximum that still works
  • Note: ethereum_hashing::ZEROHASHES ends at depth 48, so won't work for larger lists on 32-bit architectures. This could be solved in this repo by constructing zerohashes independently, or there by expanding the ZEROHASHES to 64 levels.

variable_lists.rs mostly contains uses for to_usize helper + tests for the whole deal