UserAllocatedEvent implementation

UserAllocatedEvent provides mechanism for event posting and dispatching without
utilization of queue internal memory. UserAllocatedEvent embeds all underlying
event data and doesn't require any memory allocation while posting and dispatching.
All of these makes it cannot fail due to memory exhaustion while posting.

Author: maciejbocianski

events/EventQueue.h

@@ -45,6 +45,8 @@ namespace events {
 // Predeclared classes
 template <typename F>
 class Event;
+template <typename F, typename A>
+class UserAllocatedEvent;
 
 /**
  * \defgroup events_EventQueue EventQueue class
@@ -1071,6 +1073,8 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
 #if !defined(DOXYGEN_ONLY)
     template <typename F>
     friend class Event;
+    template <typename F, typename A>
+    friend class UserAllocatedEvent;
     struct equeue _equeue;
     mbed::Callback<void(int)> _update;
 
@@ -1095,7 +1099,7 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
     struct context<F> {
         F f;
 
-        context(F f)
+        constexpr context(F f)
             : f(f) {}
 
         template <typename... ArgTs>
@@ -1110,7 +1114,7 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
         F f;
         C0 c0;
 
-        context(F f, C0 c0)
+        constexpr context(F f, C0 c0)
             : f(f), c0(c0) {}
 
         template <typename... ArgTs>
@@ -1126,7 +1130,7 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
         C0 c0;
         C1 c1;
 
-        context(F f, C0 c0, C1 c1)
+        constexpr context(F f, C0 c0, C1 c1)
             : f(f), c0(c0), c1(c1) {}
 
         template <typename... ArgTs>
@@ -1143,7 +1147,7 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
         C1 c1;
         C2 c2;
 
-        context(F f, C0 c0, C1 c1, C2 c2)
+        constexpr context(F f, C0 c0, C1 c1, C2 c2)
             : f(f), c0(c0), c1(c1), c2(c2) {}
 
         template <typename... ArgTs>
@@ -1161,7 +1165,7 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
         C2 c2;
         C3 c3;
 
-        context(F f, C0 c0, C1 c1, C2 c2, C3 c3)
+        constexpr context(F f, C0 c0, C1 c1, C2 c2, C3 c3)
             : f(f), c0(c0), c1(c1), c2(c2), c3(c3) {}
 
         template <typename... ArgTs>
@@ -1180,7 +1184,7 @@ class EventQueue : private mbed::NonCopyable<EventQueue> {
         C3 c3;
         C4 c4;
 
-        context(F f, C0 c0, C1 c1, C2 c2, C3 c3, C4 c4)
+        constexpr context(F f, C0 c0, C1 c1, C2 c2, C3 c3, C4 c4)
             : f(f), c0(c0), c1(c1), c2(c2), c3(c3), c4(c4) {}
 
         template <typename... ArgTs>