All values are now references into the value table, so drop the
distinction between direct and table values. Direct values don't exist
any more.
Also remove the parser support for the 'vxNN' syntax. Only 'vNN' values
can be parsed now.
This makes it possible to refer to entities defined in the source file,
using the source names prefixed with $.
For example, $v20 refers to the value by that name in the sources, even
if it was renumbered to 'vx0' in the parsed file.
Create a new directory hierarchy under 'filetests' for all the tests
that are run by 'cton-util test'.
Convert the parser tests under 'tests/parser' to use 'test cat' and
filecheck directives.
