Restrict the usage of the (former) string table to resource names.

Addresses bug #116.

tl;dr all resource names are strings in `motifd`, but not all strings
are resource names. To whatever degree it's useful to communicate
resource names by index, use a different code path for that than for
strings.

Suggested bottom-up reading order:

src/motif/lisp/string-base.lisp: change the Lisp-side global table
into a table of symbols, and encapsulate that table in a couple
interfaces for the rest of the system.

src/motif/lisp/internals.lisp: remove the old string table variable &
declare the interfaces from string-base.lisp as forward declarations
for the rest of the system.

src/motif/lisp/xt-types.lisp: add "toolkit type" name,
:RESOURCE-NAME, and get rid of :STRING-TOKEN.

src/motif/lisp/conversion.lisp: This is the heart of the matter. Add a
case to TOOLKIT-WRITE-VALUE for :RESOURCE-NAME: resource names are
written using MESSAGE-WRITE-RESOURCE-NAME. Change
MESSAGE-WRITE-RESOURCE-LIST and MESSAGE-WRITE-RESOURCE-NAMES to call
MESSAGE-WRITE-RESOURCE-NAME. Remove any "tokenization" from
MESSAGE-WRITE-STRING. Add a case to TOOLKIT-READ-VALUE for
:RESOURCE-NAME, & remove the case for :STRING-TOKEN. As far as Lisp is
concerned above the level of this file's abstractions, there are only
resource name symbols, no tokens, no strings.

src/motif/server/global.h: add a custom "resource representation name"
for resource names, ExtRResourceName; and a fake one for resource
names' indices in the global table. Also kill the representation name
for string tokens.

src/motif/server/tables.h and src/motif/server/tables.c: add
resource_name_tag, remove string_token_tag. Rename the string_table to
resource_name_table. Remove tokenize_string (nobody needs it).

src/motif/server/server.c: add resource_name_tag's initialization,
remove string_token_tag.

src/motif/server/datatrans.h and src/motif/server/datatrans.c: the
counterparts of the changes in conversion.lisp. Add
message_read_resource_name, and get toolkit_read_value to use it by
supplying ExtRResourceName in message_read_resource_list and
message_read_resource_names. Add error stub for writing resource
names (nothing needs to write them back to Lisp). Remove "tokenizing"
from the I/O path for strings. Get rid of the string_token I/O
functions, but add a case in toolkit_read_value for the one code path
where string_token was important (in callbacks.c, described below).

src/motif/lisp/prototypes.lisp: now that there's a "toolkit type" of
:RESOURCE-NAME, use it as an annotation in any DEF-TOOLKIT-REQUEST
forms that need it. (%GET-VALUES and %SET-VALUES already used
:RESOURCE-LIST and :RESOURCE-NAMES respectively, so the changes in
conversion.lisp take care of those.

src/motif/lisp/widgets.lisp: because conversion.lisp now knows how to
turn resource name symbols into indices directly,
CONVERT-RESOURCE-LIST and CONVERT-RESOURCE-NAMES, and any other
runtime conversion of a resource name symbol into a string goes away.

src/motif/lisp/callbacks.lisp: because conversion.lisp now knows how
to turn resource name symbols into indices directly, runtime
conversion of resource name symbols into strings goes away. (And so
most of the lines of diff in this file are just renaming arguments
from SYM-NAME to NAME, since there's no longer a non-symbol name.)

src/motif/server/callbacks.c: this is the trickiest bit. (Context:
when setting/removing/invoking callbacks, motifd uses the index of the
callback list name, rather than the name itself, for the callback
function's client_data. This is presumably an optimization: to avoid
having to looking up a name when the callback gets invoked. This
appears to have been the only reason the string_token "toolkit type"
was present; since the uses of these indices is so contained, it
seemed simplest to get rid of that type entirely.) In RXtAddCallback
and RxtRemoveCallback, replace the use of the resource representation,
ExtRStringToken, with ExtRResourceNameIndex, so that
toolkit_read_value will return the index of a resource name sent from
Lisp, rather than the resource name itself; and in CallbackHandler,
bypass the toolkit_write/message_write layer, and instead tag and put
the resource name index directly.

src/motif/lisp/interface-build.lisp: use the interfaces added to
string-base.lisp for building the C-side header file, which is now
named ResourceNameTable.h.

src/motif/server/GNUmakefile: change dependency from StringTable.h to
ResourceNameTable.h.
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