Malformed 'b' signal data can be generated

[ghost]

Hello! Thanks for making this awesome lib! I have an application which generates an malformed vcd using the interface, which I found surprising.

sbasu@ubuntu:~/Devel/rust-hdl2/rust-hdl/rust-hdl$ gtkwave ./i2c_driver.vcd 

GTKWave Analyzer v3.3.86 (w)1999-2017 BSI

[0] start time.
[100000000] end time.
GTKWAVE | malformed 'b' signal data for 'uut.driver.scl' after time_idx = 1

Here is an example of line in the .vcd file that I think causes the problem:

bz "

I suspect that the problem is that I am writing a single (scalar) value to a vector. I can probably fix this in the application, but if you are interested, I can submit a PR to handle it in the library as well.

[kevinmehall]

You probably want to use change_scalar instead of change_vector. The Writer doesn't keep track of the types of the variables you create. I'm not sure there's anything inherently wrong with a vector of width 1 according to the VCD spec, though.

[ghost]

Yeah - I changed the app to use change_scalar instead. I'm not sure why a scalar vector doesn't work. It seems particular for the use of tri-state logic though.