Created function to load symbols from source.

Created function to load symbols from source.

Function load_symbols adds built-in symbols from code
then looks through all input source for symbols. These
are added to the symbol hash table.

Maximum symbol length has been set to 256. This should
be plenty long for normal use.

NOTE: This version has a bug.
When the symbol table already contains a symbol at that
hash, the add_entry function should look through table for
a place to put the next symbol.  It does not do this
correctly, and therefore does not really fill up the
hash table.