Fix non-deterministic build (use paste! instead of gensym!)

[nanocryk]

What does it do?

In the ERC20 balances precompile we impl storage prefixes for each possible instance of pallet_balances. To avoid name clashes and long macro lines we make a dedicated module for each instance in which we create the storage prefix struct.

Before I used gensym! to generate a unique name for the module, but it's non-deterministic and thus cause issues with sr-tool.
This PR uses paste! instead to generate a module name from the instance struct name, providing a unique but deterministic name.

What important points reviewers should know?

Is there something left for follow-up PRs?

Maybe concatenate the literal automatically, and even loop throught 0 to 16 to avoid calling the macro multiple times?

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

[crystalin]

Isn't a migration required for this ?

[librelois]

Isn't a migration required for this ?

No, it's compile-time stuff only

[nanocryk]

Isn't a migration required for this ?

It doesn't change any storage key. The only change is the name of the Rust module containing the struct, which have no impact on the storage. However for building the previous code generated different module names each time you compiled, which broke sr-tool.