sputnikvm: Problem: sputnikvm users don't want to download large gaslighter dependencies

Cargo still doesn’t support bin dependencies (https://github.com/rust-lang/cargo/issues/1982). However, several dependencies we currently have (like JSON parser) are only for the bin target.

Until that, I suggest we separate sputnikvm and gaslighter into two different crates. This basically creates a structure like below:

sputnikvm/Cargo.toml
sputnikvm/src/mod.rs
sputnikvm/src/vm/..
gaslighter/Cargo.toml
gaslighter/src/mod.rs

The sputnikvm will then be used to build the library, and the gaslighter will be used to build the executable.