Fix _encode_identifier to always emit valid identifiers

[dakerfp]

No description provided.

[jpgrayson]

From ieee 1364-2001:

The value change dumper generates character identifier codes to represent variables. The identifier code is a
code composed of the printable characters which are in the ASCII character set from ! to ~ (decimal 33 to
126).

This PR changes VCDWriter such that identifier codes always start with A-z, but the spec does not require this.

Is there some application that has trouble with [standards compliant] base-94 identifier codes? What is this fixing?

[dakerfp]

pyvcd.reader itself doesn't work with identifier codes starting out of the A-z range.