Use base-94 encoded ASCII identifiers

This is an efficient and perfectly hashable encoding for variable
identifier codes. It is based on Tony Bybell's algorithm from
"Implementation of an Efficient Method for Digital Waveform Compression"
which can be found as an appendix in the GTKWave documentation.

The new encoding will make VCD files containing many variables smaller
since each change line will require fewer ascii characters for the
identifier than with the previous base-16 (hexadecimal) encoding.

README.rst

@@ -27,27 +27,33 @@ Quick Start
    >>> from vcd import VCDWriter
    >>> with VCDWriter(sys.stdout, timescale='1 ns', date='today') as writer:
    ...     counter_var = writer.register_var('a.b.c', 'counter', 'integer', size=8)
+   ...     real_var = writer.register_var('a.b.c', 'x', 'real', init=1.23)
    ...     for timestamp, value in enumerate(range(10, 20, 2)):
    ...         writer.change(counter_var, timestamp, value)
+   ...     writer.change(real_var, 5, 3.21)
    $date today $end
    $timescale 1 ns $end
    $scope module a $end
    $scope module b $end
    $scope module c $end
-   $var integer 8 0 counter $end
+   $var integer 8 ! counter $end
+   $var real 64 " x $end
    $upscope $end
    $upscope $end
    $upscope $end
    $enddefinitions $end
    #0
    $dumpvars
-   b1010 0
+   b1010 !
+   r1.23 "
    $end
    #1
-   b1100 0
+   b1100 !
    #2
-   b1110 0
+   b1110 !
    #3
-   b10000 0
+   b10000 !
    #4
-   b10010 0
+   b10010 !
+   #5
+   r3.21 "