Switch Hex helper from message passing over `stdio` to using either the new Hex CLI or IPC

[jeffwidman]

Code improvement description

Described in detail here:

• Bump Hex to 2.0.1 #6026 (comment)
• Bump Hex to 2.0.1 #6026 (comment)
• Bump Hex to 2.0.1 #6026 (comment)

Also related:

• Propagate mixfile errors to the user better. #2757
• Bump Hex package manager to 2.0 #6009

Filing this ticket mostly so we don't lose track of it as a source of potential improvements.