Make static context const

[purpleKarrot]

Fixes #1637

[real-or-random]

Looks good to me.

The only caveat is that this is, strictly speaking, a breaking change. If we want to be strict, we'd need a 0.7.0 instead of a 0.6.1. But I don't think we care?! I mean, any user code writing to the variable is most likely buggy... Hm.

[sipa]

Technically speaking, this is certainly an API change. A constant and non-constant variable are not compatible as far as the C language is concerned. A (buggy) program could fail to compile with this change, while it was fine before.

But is it an ABI break? The symbol still exists, and has the same size and meaning. Linking will not fail for any reason, it is only at runtime that an attempt to modify the variable may (or may not) trigger a memory violation.

[real-or-random]

it is only at runtime that an attempt to modify the variable may (or may not) trigger a memory violation.

The good thing is that a clean crash is probably the worst that can happen in practice...

I think I'm okay with anything here. If you ask me, we can treat this as a breaking or a non-breaking chance, when it comes to both API and ABI.

[real-or-random]

In any case, I prefer this proper fix over introducing a second symbol just to keep backwards compatibility.

utACK bfc3ae0

We can debate at release time whether this is breaking or not.