boost: remove CPubKey dependency from CECKey. Follow-up of e405aa48
boost: code movement only: split CECKey into separate files
Merge pull request #5118

50f71cd boost: code movement only: split CECKey into separate files (Cory Fields)
bdaec6a boost: remove CPubKey dependency from CECKey. Follow-up of e405aa48 (Cory Fields)