removed now-obsolete include

Was rendered obsolete by the last commit.
fixed documentation

Added quite a few documentation comments and fixed up existing ones.
removed obsolete file

SourceManager-interal.hpp became obsolete with the separation of
SourceManagerEntry into its own pair of files.
srcmgr entry inits buffer to null

Changed the constructor for SourceManagerEntry to initialize its buffer to a
nullptr. Just to make sure things go haywire fast if someone tries to use an
unassigned buffer.
progress towards nice diagnostic msgs

DiagnosticFormatterConsole is getting close to printing highly polished
messages that include source code snippets and fixit messages. Soon.