Support `char [N]` assignment from string

[vsergeev]

local ffi = jit and require('ffi') or require('cffi')

ffi.cdef[[
struct foo {
    char str[4];
};
]]

local f = ffi.new("struct foo")

f.str = "abc" --> expected: no error, get: "invalid C type"
print(ffi.string(f.str)) --> abc

f.str = "abcd1234"
print(ffi.string(f.str)) --> abcd