Source Error Slots

[jbouwman]

Normalize source condition notes by merging source-condition class and source-error struct.

• A condition has a string-valued message, and zero or more notes, generally one.
• A note is minimally a string and a location

Improve the checking of type inference errors: look for the exact text, rather than the condition.

[jbouwman]

generic function definitions here have been moved to separate PRs, other portions are obsolete