Add const representation for standard header names

[caspervonb]

Add const representation (an enum) for standard known header names.

Closes #945

[Jarema]

It looks ok, but I'm not happy about reintroducing unsafe block.
Wondering if we can get rid of it.

[caspervonb]

It looks ok, but I'm not happy about reintroducing unsafe block.
Wondering if we can get rid of it.

Can't really get rid of it. It is safe tho, as the data is valid utf-8.

[Jarema]

Can't really get rid of it. It is safe tho, as the data is valid utf-8.

@caspervonb I agree it's safe, but as some of our contributors mentioned, people run checks and disallow libraries with unsafe blocks. That's my concern.

[caspervonb]

@caspervonb I agree it's safe, but as some of our contributors mentioned, people run checks and disallow libraries with unsafe blocks. That's my concern.

I'm not particularly worried about invariants that are known to be safe.
Even basic std string operations contain unsafe blocks.

Difference between just having to call an unsafe function to achieve an operation that is safe for a certain condition and being clever with unsafe hacks are quite different imo.

[Jarema]

LGTM