string to/from parameter conversion

[wrl]

VST3 and AU support arbitrary conversion of strings to and from parameter values, which are not necessarily the current value of the parameter. this is useful in situations where a user is adjusting automation points – without the ability to get the string representation of an arbitrary normalised value for a given parameter, hosts can only display the raw normalised value.

some of this work will land with the VST3 support in #5, but parameter::Format::display_cb needs some retooling to support this. in particular, display_cb only uses the current parameter value, and should be modified to take an arbitrary value in.