Add typedef's for MPS buffer size types

Most buffers that MPS deals with are small and representable
with integer types of width 16-bit or more.

For highly memory constrained systems, it is therefore a potential
for significant memory savings to use 16-bit types for buffer sizes
throughout MPS.

In prepraration for this, this commit introduces typdefs

```
   mbedtls_mps_size_t
   mbedtls_mps_stored_size_t
```

for buffer sizes in the MPS implementation and the MPS structures,
respectively.

So far, those MUST be defined as `size_t`: While an effort has been made
to write most of MPS code in terms of `mbedtls_mps_[stored_]size_t` in a
way that would allow narrower types, those aren't yet supported. Still,
we retain the typedefs in order to avoid unnecessary rewriting of a large
body of the MPS codebase.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

library/mps/common.h

@@ -26,6 +26,8 @@
 #ifndef MBEDTLS_MPS_COMMON_H
 #define MBEDTLS_MPS_COMMON_H
 
+#include <stdio.h>
+
 /**
  * \name SECTION:       MPS Configuration
  *
@@ -52,4 +54,55 @@
 
 /* \} name SECTION: MPS Configuration */
 
+/**
+ * \name SECTION:       Common types
+ *
+ * Various common types used throughout MPS.
+ * \{
+ */
+
+/** \brief   The type of buffer sizes and offsets used in MPS structures.
+ *
+ *           This is an unsigned integer type that should be large enough to
+ *           hold the length of any buffer resp. message processed by MPS.
+ *
+ *           The reason to pick a value as small as possible here is
+ *           to reduce the size of MPS structures.
+ *
+ * \warning  Care has to be taken when using a narrower type
+ *           than ::mbedtls_mps_size_t here because of
+ *           potential truncation during conversion.
+ *
+ * \warning  Handshake messages in TLS may be up to 2^24 ~ 16Mb in size.
+ *           If mbedtls_mps_[opt_]stored_size_t is smaller than that, the
+ *           maximum handshake message is restricted accordingly.
+ *
+ * For now, we use the default type of size_t throughout, and the use of
+ * smaller types or different types for ::mbedtls_mps_size_t and
+ * ::mbedtls_mps_stored_size_t is not yet supported.
+ *
+ */
+typedef size_t mbedtls_mps_stored_size_t;
+#define MBEDTLS_MPS_SIZE_MAX  ( (mbedtls_mps_size_t) -1 )
+
+/** \brief The type of buffer sizes and offsets used in the MPS API
+ *         and implementation.
+ *
+ *         This must be at least as wide as ::mbedtls_stored_size_t but
+ *         may be chosen to be strictly larger if more suitable for the
+ *         target architecture.
+ *
+ *         For example, in a test build for ARM Thumb, using uint_fast16_t
+ *         instead of uint16_t reduced the code size from 1060 Byte to 962 Byte,
+ *         so almost 10%.
+ */
+typedef size_t mbedtls_mps_size_t;
+
+#if (mbedtls_mps_size_t) -1 > (mbedtls_mps_stored_size_t) -1
+#error "Misconfiguration of mbedtls_mps_size_t and mbedtls_mps_stored_size_t."
+#endif
+
+/* \} SECTION: Common types */
+
+
 #endif /* MBEDTLS_MPS_COMMON_H */