specl: refetch Lua fragments without line-folding for compilation.

* lib/specl/loader.lua (Parser): Save the input string, so we can
grab parts of it when refetching Lua fragments.
(parser_mt:refetch): New method to refetch the Lua code fragment
without line-folding, using offsets in the libYAML marker indices.
(parser_mt:load_scalar): Add the scalar event table to the return
statement.
Don't save the style into the current event.
(parser_mt:load_map): Use refetch method with returned scalar
event, and pass that unfolded fragment to parser_mt:compile.
(parser_mt:compile): No need to conditionally fiddle with the line
number in error messages.
* specs/specl_spec.yaml (with Lua error in multi-line example):
Adjust to expect correct line numbers.
* specs/optparse_spec.yaml: Replace unicode characters with ASCII
equivalents.
* NEWS: Warn that unicode characters mess up libYAML indices.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>