uri: replace static_alloc with ffi stash and ibuf
static_alloc() appears not to be safe to use in Lua, because it does not provide any ownership protection for the returned values. The problem appears when something is allocated, then Lua GC starts, and some __gc handlers might also use static_alloc(). In Lua and in C - both lead to the buffer being corrupted in its original usage place. The patch is a part of activity of getting rid of static_alloc() in Lua. It removes it from uri Lua module and makes it use the new FFI stash feature, which helps to cache frequently used and heavy to allocate FFI values. In one place static_alloc() was used for an actual buffer - it was replaced with cord_ibuf which is equally fast when preallocated. ffi.new() for temporary struct uri is not used, because - It produces a new GC object; - ffi.new('struct uri') costs around 20ns while FFI stash costs around 0.8ns. The hack with 'struct uri[1]' does not help because size of uri is > 128 bytes; - Without JIT ffi.new() costs about the same as the stash, not better as well; The patch makes uri perf a bit better in the places where static_alloc() was used, because its cost was around 7ns for one allocation.
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