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.

[sffc]

@robertbastian I still have a lot on my plate for 2.0. Would you mind taking a look at this PR, reproducing and possibly fixing the binary size regression shown above?

[robertbastian]

tutorials_buffer.wasm is now at 243150

Edit: due to the rebase the numbers from Shane's post are no longer accurate. New baselines

Commit	Size
Main	242744
Add data marker hash	240770
Remove data marker path	244179
Simplify Debug impl	244847
...	...
265e9c4	243150
attributes domain	242321

[sffc]

Your latest changes look good; thanks. Still no idea why the binary size changes in the opposite direction than expected.

How do you suggest we proceed?

[robertbastian]

I will stare at this more.

[sffc]

Replaced by #5981