Store data path hash instead of string in binary

[sffc]

Proof-of-concept for #4991

[sffc]

@Manishearth What do you think about this approach?

The actual path strings are used in two places:

1. Error messages, including impl Debug
2. FsDataProvider

This PR makes the Debug impl use the hash and FsDataProvider get the path from the registry.

One thing that puzzles me is the size of tutorials_buffer.wasm.

Commit	Description	Size
9e5a25b	Main	250697
68fcc7e	Add data marker hash	246851
f8d1505	Remove data marker path	251087
8f34fd5	Simplify Debug impl	251888

My expectation is that the size should go up on row 2 and down on rows 3 and 4, but it is basically the opposite of my expectation. I've inspected the assembly and I have no explanation for this.

[Manishearth]

I like this, as long as datagen is able to speak in terms of the regular paths and debugging hashes is not too hard. Which I think will be fine with the registry.

My expectation is that the size should go up on row 2 and down on rows 3 and 4, but it is basically the opposite of my expectation. I've inspected the assembly and I have no explanation for this.

Is the path included in the binary elsewhere? I bet something else includes it so it gets reused.

[sffc]

Briefly discussed in the WG call. @Manishearth and @hsivonen are supportive of the change, and @sffc will continue to investigate the binary size issue.