Consider building rust contracts with cross

[blckngm]

Pros:

• More familiar for rust devs.
• Smaller images (no rust).
• Can use any rust version.
• Better support for cargo commands and options, e.g. cross build -v, cross clippy etc (#35).
• Easier to maintain. capsule build can be a thin wrapper of cross.

~It mostly already works, I had to make one small change to cross:~

~https://github.com/cross-rs/cross/commit/c632612f99f7387d18d8eac0550b2a96596e1f2e~

Add this configuration (Cross.toml) to a capsule project:

[build]
default-target = "riscv64imac-unknown-none-elf"

[target.riscv64imac-unknown-none-elf]
# Both image works
# image = "thewawar/ckb-capsule:2022-08-01"
image = "nervos/ckb-riscv-gnu-toolchain:focal-20230214"

Then I can cross +nightly build -p some_contract

[blckngm]

Shouldn't break reproducible build as long as the same rust version, options and image is used.

[blckngm]

Cons:

• Users will need to install cross

[blckngm]

It works with cross main branch now.

[blckngm]

Cross also supports path deps outside of the project dir.