The compiler was coercing base strings to extended strings when
encountering them as literal objects in compiled files. According to
CLHS 3.2.4.2.2, we need to preserve the actual element type (after
upgrading) of arrays during compilation. For ECL, the actual array
element type of a base-string is base-char and therefore we can't
change this to an extended string.
Actually fixing this requires some work since we use the reader to
store a printed representation of the string in the compiled file. The
reader string syntax always returns an extended string (There is a
comment in the code which claims that this is implied by CLHS 2.4.5. I
am not quite sure if that is really true but for backwards
compatibility I don't want to change this). We thus introduce a new
syntax #"..." for base strings which is used when reading objects from
compiled files. To prevent the new syntax from leaking outside of this
context, we also introduce a new readtable.
