Use std::ffi::{c_char, c_float} instead of libc's

They're equivalent. Unfortunately std::ffi::c_size_t is currently experimental, and if I define c_size_t = usize myself then cbindgen doesn't recognise it properly and makes c_size_t a typedef of uintptr_t instead of just replacing it with size_t.

ffi/src/common.rs

@@ -1,6 +1,6 @@
-use std::ffi::CString;
+use std::ffi::{c_char, CString};
 
-use libc::{c_char, size_t};
+use libc::size_t;
 
 /// Free the memory allocated for the given string.
 ///

ffi/src/helpers.rs

@@ -1,6 +1,6 @@
-use std::ffi::{CStr, CString};
+use std::ffi::{c_char, CStr, CString};
 
-use libc::{c_char, size_t};
+use libc::size_t;
 
 use esplugin::{GameId, Plugin};
 

ffi/src/plugin.rs

@@ -1,7 +1,8 @@
+use std::ffi::{c_char, c_float};
 use std::path::Path;
 use std::{f32, panic, ptr};
 
-use libc::{c_char, c_float, size_t};
+use libc::size_t;
 
 use esplugin::{ParseOptions, Plugin, PluginMetadata};