Disallow accidental change of tag commands that share representation.

Disallow accidental change of tag commands that share representation.

After inserting commands into common index some of them may share
representations in memory. Initially commands are stored in mutable
lists, which is very convenient for tag optimizations. However,
leaving this representation after indexing is dangerous, because
accidental modification of one command may affect other commands.

This commit hides mutable lists behind indices in hash table.
Table gives read-only access to the lists.

Table index is 32-bit, which is smaller than the previous 'index'
type (a pair of pointers). So in theory command comparison has
become a little faster.