Switch to linked prop for spec lists

[muzimuzhi]

Currently a spec list stores

• spec keys in \g__tblr_<spec>_<level>_clist
• and spec values in \g__tblr_spec_<level>_<spec>_<key>_tl

As spec keys come from both prop keys (always str) and running code (using current catcode), duplicate keys may not be removable (when tracing), unless we convert all keys to str:

 \cs_new_protected:Npn \__tblr_spec_gput:nnn #1 #2 #3
   {
     \tl_if_exist:cF { g__tblr_spec_ \int_use:N \g_tblr_level_int _#1_#2_tl }
       {
         \tl_new:c { g__tblr_spec_ \int_use:N \g_tblr_level_int _#1_#2_tl }
       }
     \tl_gset:cn
       { g__tblr_spec_ \int_use:N \g_tblr_level_int _#1_#2_tl } {#3}
-    \clist_gput_right:ce { g__tblr_#1_ \int_use:N \g_tblr_level_int _clist } { #2 }
+    \clist_gput_right:ce { g__tblr_#1_ \int_use:N \g_tblr_level_int _clist } { \tl_to_str:e {#2} }
   }

or just

\clist_gput_right:co { g__tblr_#1_ \int_use:N \g_tblr_level_int _clist } { \tl_to_str:n {#2} }

if #2 is ensured unexpandable.

Linked property list variables (which needs l3kernel 2024-02-13, better 2024-02-18)

• have all the virtue of (old, flat) props: no duplicate keys, ensured str key
• take constant time for if_in, item, put/get, pop/remove operations.