no error for over-long character literal

[JeffBezanson]

I get

julia> 'abc'
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)

Should be

julia> 'abc'
ERROR: syntax: character literal contains multiple characters

[c42f]

This is part of a broader problem with diagnostics not being present in validating string->value conversions. The general fix for these kind of issues will be something like #82 (comment)

[c42f]

This was somewhat improved as a side effect of #121 but still needs a more systematic fix.

[c42f]

Also related to this - invalid escape sequences don't give a useful error.