Stop using _<name> reserved identifiers

[FranklandJack]

Unified Runtime appears to have followed the example of OpenCL and used _<name> in typedefs of the opaque API objects e.g. _ur_module_handle_t.

This is actually illegal in both the C and C++ specifications as all identifiers with the _ prefix are reserved for standard library implementation and internals.

We should rename the structs in this pattern.

[kbenzie]

I've also seen this cause errors in the Sphinx build. The reStructuredText is referring to <name> for an enum or similar but not being able to resolve it name because Doxygen has parsed the enum type name it as _<name>, <name> actually refers to a typedef.

[pbalcer]

I was looking over #23 and also noticed that many identifiers use _t suffix (often for both struct definition and in the typedef), which is reserved by POSIX. Not sure if this is worth changing though, since conflicts are very unlikely.

glibc docs have a handy list of reserved names: https://www.gnu.org/software/libc/manual/html_node/Reserved-Names.html

[FranklandJack]

I think the _t is a good point, but like you say conflicts are very unlikely. Adapters could in theory include POSIX headers, but I think the probability of them defining something like ur_context_handle_t is so low that we can probably ignore this issue for the time being. Worth bearing in mind though in case it does come up at some point.