Improve file structure and crate name

[webmaster128]

Right now we have two main crates cosmwasm and cosmwasm_vm with cosmwasm_vm depends on cosmwasm. However, the file structure does not reflect that very well, since cosmwasm_vm is in a subfolder ./lib/vm of cosmwasm (which is in .), which suggests the VM is part of cosmwasm.

Also the name cosmwasm does not specify what the crate actually does. It is somewhat everywhere but I find it hard to reason about what belongs in there.

To improve this situation, I suggest 2 changes:

1. Separate the workspace root Cargo.toml from the cosmwasm crate's Cargo.toml by moving the package into ./lib/std, next to the VM.
2. Rename the crate cosmwasm to cosmwasm_std, as it is the standard library that is available for development inside of contracts. This should make clear that everything that is not required for contract development does not belong here.

The first step can be considered compatible I guess. The second one could be scheduled for 0.8.

[ethanfrey]

Sounds good.

Yes, please do (1) on master, keeping same release name, so there are less merge issues with 0.8.

Then you can start a 0.8 branch and do (2)

[webmaster128]

I was thinking of using cosmwasm_std and cosmwasm_vm instead of the hyphen separated variant, as it would give us the same spelling in the crate name as well as the import. Unfortunately it is not possible to rename the existing cosmwasm_vm to cosmwasm-vm on crates.io.

I guess we stick with hyphens consistently as long as the renameing is not supported.

[ethanfrey]

The standard in rust is to prefer foo-bar in the crate names while publishing.
This is auto-converted to foo_bar in imports.

I'd stick with this as it fits with the aesthetics of the rust community.