Version serialized files

[andrewdavidmackenzie]

add a version number for the file format for files:

• manifest file between flowc and flowrunners in particular
• maybe flow specs too

should be pretty simple I think, but maybe some crates like

• https://crates.io/crates/versionize
• https://github.com/fredpointzero/serde-version

can automatically do it and catch changes in underlying structs and automatically increment version number?