World names can collide with GLSL keywords

[tangent-vector]

I tried to name my frament world fragment and so forth, but when I try to name a world uniform, the Spire front-end generates GLSL code with a declaration like:

uniform uniform { ... }

which is obviously not valid GLSL. I haven't done any digging into whether using GLSL keywords for other sorts of names causes problems, but it would be nice if the output GLSL went to some effort to "mangle" names to avoid collision with GLSL keywords.

A simple strategy would be to prefix every name with something like SPIRE_*, but this would result in less immediately readable GLSL output.