Remove Buffer::Pointer & ConstPointer

The Pointer and ConstPointer type aliases are confusing because their
meaning does not match with that of STL containers.
Rename Buffer.cc/hh to buffer.cc/hh
Adopt STL naming convention in buffer.hh
Adopt STL naming convention in DiagnosticMessage.hh
Rename Source.cc/hh to source.cc/hh
Rename Source to source
Adopt STL naming convention in source.hh
Adopt STL naming convention in source.cc
Adopt STL naming convention in source_test_helper.hh
Adopt STL naming convention in LineAppendedSource.cc/hh
Rename LineContinuedSource to line_continued_source
Rename Location.cc/hh to location.cc/hh
Rename Location to location
Rename LineLocation to line_location
Adopt STL naming convention in location.cc/hh
Adopt STL naming convention in Origin.hh
Merge pull request #128 from magicant/refactor/rename

Adopt STL naming convention in sesh::language::source