Allow explicit widths in project/platform symbols, part 4 (of 4)

Handle situation where a symbol wraps around a bank.  Updated
2021-external-symbols for that, and to test the behavior when file
data and an external symbol overlap.

The bank-wrap test turned up a bug in Merlin 32.  A workaround has
been added.

Updated documentation to explain widths.

SourceGen/RuntimeData/Help/editors.html

@@ -302,11 +302,19 @@ with platform symbols and user labels.</p>
 <p>The value may be entered in decimal, hexadecimal, or binary.  The numeric
 base you choose will be remembered, so that the value will be displayed
 the same way when used in a .EQ directive.</p>
+<p>You can optionally provide a width.  For example, if the address is
+of a two-byte pointer or a 64-byte buffer, you would set the width field
+to cause all references to any location in that range to be set to the
+symbol.  Widths may be entered in hex or decimal.  If the field
+is left blank, a width of 1 is assumed.  Overlapping symbols are allowed.
+The width is ignored for constants.</p>
 <p>If you enter a comment, it will be placed at the end of the line of
 the .EQ directive.</p>
 <p>Symbols marked as "address" will be applied automatically when an
-operand references an address outside the scope of the data file.  Symbols
-marked as "constant" will not, though you can still specify them manually.</p>
+operand references an address outside the scope of the data file.  They
+will not be applied to addresses inside the data file.  Symbols
+marked as "constant" are not applied automatically, and must be
+explicitly specified as an operand.</p>
 
 
 <h2><a name="lvtable">Create/Edit Local Variable Table</a></h2>