Cache C interface

[dpino]

It's a common idiom in Lua to cache the C interface:

local ffi = require('ffi')
local C = ffi.C

Caching the C namespace allows faster access to C functions. The codebase of Snabbswitch uses this idiom but there are a few places which don't, even when the C namespace was previously cached.

[dpino]

Performance notice: the JIT compiler specializes to the identity of namespace objects and to the strings used to index it. This effectively turns function cdata objects into constants. It's not useful and actually counter-productive to explicitly cache these function objects, e.g. local strlen = ffi.C.strlen. OTOH it is useful to cache the namespace itself, e.g. local C = ffi.C.

Source: http://luajit.org/ext_ffi_semantics.html