Separate migrations from util

[debris]

based on #6573
closes #6418

In #6573 @Hawstein wrote:

separate migration from util and make its dependencies into libs:

• snappy
• kvdb
• error
• common

I applied few additional changes:

• renamed /common -> macros, cause this crate provides only macros
• renamed ethcore_error -> util_error, cause the name is misleading
• kvdb does not use util_error any longer. instead it provides it's own error type
• util_error no longer depends on snappy. conversion from snappy error was not used anyway
• merged nibbleslice and nibblesvec into patricia_trie crate. there is no need to keep them separated
• removed unused imports

TODO (will open issue accordingly, after this pr is merged):

• kvdb should be separated into 2 separate crates: kvdb and kvrocksdb which implements key-value interface for rocksdb
• journal crate should be separated from util. overlaydb and util_error crate should be merged into it. also UtilError should be just renamed to Error.
• cache should be separated from util
• util should be dissolved