Add string helper function to Innodb::Record
Add a reasonable Innodb::System implementation

Significantly expand Innodb::DataDictionary and build a reasonably
complete Innodb::System implementation based on it. Some additional
work and refactoring is done in innodb_space, including:

  * A new command system-spaces to print some information about all
    spaces in the system.
  * Four new commands data-dictionary-{tables,columns,indexes,fields}
    to print the contents of the data dictionary tables with the
    respective names.
  * It is only possible to use one mode at a time, to simplify the
    processing of various commands and make documenting their effects
    more sane.
  * It is only possible to use one page number (-p), level (-l) and
    list (-L) at a time, for the same reason.

Although initial tests have been added for all methods, to ensure they
at least get exercised by RSpec, much more robust and useful tests
should still be added.