Options to elide literal types in more places about c2rust

[TheDan64]

This is a good point; there are a bunch of type annotations that don't really add value such as (non function) null pointers and struct initializers which type inference would do fine without:

let mut x: *mut MyStruct = 0 as *mut MyStruct;
let mut y: MyStruct = MyStruct { ... };

[TheDan64]

@anp I've added a flag --reduce-type-annotations as of 14223c6 to the ast-importer which can be used for this purpose. Here is an example of local variables, some of which will have no explicit type annotation when the flag is passed:

int a;
int b = 0;
int c = foo();
struct MyStruct d = {0};
int *e = 0;

which becomes:

let mut a: libc::c_int = 0;
let mut b = 0i32;
let mut c = foo();
let mut d = MyStruct{i: 0i32,};
let mut e = 0 as *mut libc::c_int;