Allow concurrent live Sessions

The existing bindings require a `&mut` reference to the context,
indicating that only one session may be alive at a time per context.
However, GlobalPlatform v1.0 says:

However, it is valid for the Client Application to concurrently use
these functions to initialize or finalize different objects; in the
above example two threads could initialize different TEEC_Session
structures.

This indicates both:
1. It should be possible to have two simultaneous live sessions.
2. If synchronization is necessary on the `TEE_Context` object, it is
   the responsibility of the C library to provide that synchronization.

This means that we do not require exclusive access to the `TEE_Context`,
either at creation time or during use of the session, so we can take a
`&` instead.

optee-teec/src/context.rs

@@ -68,8 +68,8 @@ impl Context {
     /// let mut ctx = Context::new().unwrap();
     /// let mut raw_ptr: *mut optee_teec_sys::TEEC_Context = ctx.as_mut_raw_ptr();
     /// ```
-    pub fn as_mut_raw_ptr(&mut self) -> *mut raw::TEEC_Context {
-        &mut self.raw
+    pub(crate) fn as_mut_raw_ptr(&self) -> *mut raw::TEEC_Context {
+        & self.raw as *const _ as *mut _
     }
 
     /// Opens a new session with the specified trusted application.
@@ -83,7 +83,7 @@ impl Context {
     /// let uuid = Uuid::parse_str("8abcf200-2450-11e4-abe2-0002a5d5c51b").unwrap();
     /// let session = ctx.open_session(uuid).unwrap();
     /// ```
-    pub fn open_session(&mut self, uuid: Uuid) -> Result<Session> {
+    pub fn open_session(&self, uuid: Uuid) -> Result<Session> {
         Session::new(
             self,
             uuid,
@@ -106,7 +106,7 @@ impl Context {
     /// let session = ctx.open_session_with_operation(uuid, operation).unwrap();
     /// ```
     pub fn open_session_with_operation<A: Param, B: Param, C: Param, D: Param>(
-        &mut self,
+        &self,
         uuid: Uuid,
         operation: &mut Operation<A, B, C, D>,
     ) -> Result<Session> {

optee-teec/src/session.rs

@@ -43,13 +43,13 @@ pub enum ConnectionMethods {
 /// Represents a connection between a client application and a trusted application.
 pub struct Session<'ctx> {
     raw: raw::TEEC_Session,
-    _marker: marker::PhantomData<&'ctx mut Context>,
+    _marker: marker::PhantomData<&'ctx Context>,
 }
 
 impl<'ctx> Session<'ctx> {
     /// Initializes a TEE session object with specified context and uuid.
     pub fn new<A: Param, B: Param, C: Param, D: Param>(
-        context: &'ctx mut Context,
+        context: &'ctx Context,
         uuid: Uuid,
         operation: Option<&mut Operation<A, B, C, D>>,
     ) -> Result<Self> {