Add default `[profile.release]` values to Cargo.toml if not present

Created by: ascjones

As a follow up to #33, which enables amending the Cargo.toml, we could add the following section if it is not present:

[profile.release]
panic = "abort"
lto = true
opt-level = "z"
overflow-checks = true

These are the default optimal settings for contract binary size. However if any of these settings are already defined in the Cargo.toml, then those values should be used instead of these defaults. Potentially the user could be warned if using a non-optimal setting.