Auto merge of #2363 - emarteca:int-function-args-returns, r=oli-obk

Adding support for external C functions that have integer (or empty) args and/or returns

Starts addressing `@https://github.com/rust-lang/miri/issues/11`

### Implementation
Adding support for calling external C functions that have any number of integer arguments (types of integers: `i8`, `i16`, `i32`, `i64`, `u8`, `u16`, `u32`, `u64`) and an integer return type (or `void`).
As suggested in `@https://github.com/rust-lang/miri/issues/11,` the [`libffi` crate](https://docs.rs/libffi/latest/libffi/index.html) is used to dispatch the calls to external C functions.

#### Modifications
Main modifications are to:
* [helper.rs](https://github.com/emarteca/miri/blob/int-function-args-returns/src/helpers.rs) : adding a function `call_and_add_external_c_fct_to_context` to read the code pointer to the external C function, dispatch the call, and save the return in MIRI internal memory. Handles all conversions between MIRI and C values (using some macros, also defined in this file).
* [foreign_items.rs](https://github.com/emarteca/miri/blob/int-function-args-returns/src/shims/foreign_items.rs) : handles the calling of `call_and_add_external_c_fct_to_context` in [helper.rs](https://github.com/emarteca/miri/blob/int-function-args-returns/src/helpers.rs) when a foreign item is encountered. Also adds some structs to model C representations of arguments, and the signature of the external C call.

### Testing
Adds tests for the following external functions which are now supported:
* [int tests](https://github.com/emarteca/miri/blob/int-function-args-returns/tests/pass/external_C/int_c_tests.rs):
     - adds 2 to a provided int (no type of int specified, so autocasts)
     - takes the sum of its 12 arguments (tests stack spill)
     - adds 3 to a 16 bit int
     - adds an `i16` to an `i64`
     - returns -10 as an unsigned int
* [void tests](https://github.com/emarteca/miri/blob/int-function-args-returns/tests/pass/external_C/print_from_c.rs)
     - void function that prints from C

### Code review
The code in this PR was reviewed by `@maurer` on [another fork](https://github.com/maurer/miri/pull/1) -- thanks!

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