Improve icu4x_key_extract

[robertbastian]

In #1480 we use strings, which might not be available on all systems.

[sffc]

It makes me uneasy to think that a pivotal part of the ICU4X data pipeline rests on piping strings to sed. I'd rather have a tool that takes an executable and returns the key list.

[robertbastian]

I wonder if we can exploit the linker somehow. Like putting all the path strings in one section and then looking at what appears there. Unfortunately there's rust#70239

[sapriyag]

what should be the size of this issue? S-medium or S-large? Can you please add the size label to this issue.

[sffc]

For the purposes of 1.0, we want this to be well-documented and runnable by users without having to clone the icu4x project repo.