Fix ffi meta

[Lyrth]

Fix some behavior in ffi meta

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---@field [string] function

1 - Fixes below issue when C functions are accessed from namespace tables.

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---@overload fun(init?: any, ...): ffi.cdata*
---@overload fun(nelem?: integer, init?: any, ...): ffi.cdata*

2 - ctype isn't a function, it says that ffi.ctype* can be called like a constructor (see Calling a cdata object section):

Calling a cdata object
Constructor: a ctype object can be called and used as a constructor. This is equivalent to ffi.new(ct, ...), unless a __new metamethod is defined.

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---@class ffi.cb*:    ffi.cdata*

3 - cb types can only come from ffi.new or ffi.cast, e.g.: ffi.cast('int (*)()', function() return 1 end)

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---@param params? any
function ffi.cdef(def, params, ...) end

4 - ffi.cdef and ffi.typeof accepts "parameters" for cdecls containing $ (see Parameterized Types section):

Any place you can write a typedef name, an identifier or a number in a declaration, you can write $ (the dollar sign) instead. These placeholders are replaced in order of appearance with the arguments following the cdecl string

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---@overload fun(ct: ffi.ct*, init: any, ...)

5 - Prevent the following issue from happening (when nelem is skipped and init is meant to be specified):

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( Removal of ffi.ctype )
6 - see issue 2 above

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---@param params? any
---@return ffi.ctype* ctype
---@nodiscard
function ffi.typeof(ct, params, ...) end

7 - see issue 4 above

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---@nodiscard
function ffi.cast(ct, init) end

8 - ---@nodiscard is useful here because casts are always meant to be used in ffi.

[sumneko]

Thank you!