Expose HTML-Formatted Profiling Heatmaps in `futhark profile`

[Hyalunar]

Currently, exploring Profiling information takes a lot of effort.

To find out where optimisations might be necessary one must:

• benchmark the executable (enable profiling)
• pass the giant profile.json to futhark profile
• find a cost centre big enough to be interesting in profile.summary
• locate the cost centre source locations the source location listings
• find the specific pieces of code referenced by the cost centre, they can be split over multiple files too

I discussed with @athas (sorry about the tag, I wanted to credit you =] ) in the matrix channel, they had the following idea:
Let futhark profile generate HTML-Documents containing source code heatmaps.
In this heatmap, hot code could be colored, e.g. in shades of red.
It would also be possible to hyperlink from cost centres to relevant source locations (and the other way around).

This requires a web browser to use the profiling information, which is a big leap. Previously, only a text viewer was required.
However, web browsers are omnipresent nowadays. Myself, I would struggle to find a development device that does not have a web browser.

[athas]

Text-based web browsers also exist, and I use them occasionally. The HTML generated by futhark doc is quite readable in e.g. lynx. I am not going to make that a hard requirement for futhark profile, though.

[Hyalunar]

Since the futhark compiler is a heavily optimising compiler, it may happen that a single cost centre is comprised of multiple source locations.
Should the event time be distributed among them? (divide by amount of source locations)
This would keep the total time, but may skew the results when many cheap operations are fused with one costly operation.

Alternatively, every source location lists the total time of all cost centres associated with it. But then the parts may not add up exactly to the original sum.