Support use in CUDA device-side code

[eyalroz]

• Now using a TCB_CONSTEXPR macro instead of constexpr, to hook our extra decorations
• Decorating TCB_CONSTEXPR functions with __host__ __device__ when compiling CUDA code.

Notes:

1. No direct decoration necessary, since spans are fully-constexpr.
2. No decorations for functions involving exceptions, as they're host-only.
3. Not well-tested at the moment.

[eyalroz]

@tcbrindle : How about now?

Also, even if you ok this - don't merge yet.

[sethrj]

@eyalroz I had to make some changes to your tentative span to get it to compile with nvcc (some errors, some warnings).

diff --git a/span.h b/span.h
index 380402b..41a20da 100644
--- a/span.h
+++ b/span.h
@@ -52,6 +52,11 @@
 #define TCB_SPAN_CUDA_HOST_AND_DEVICE
 #endif
 
+#ifdef __CUDA_ARCH__
+#define TCB_SPAN_NO_EXCEPTIONS
+#define TCB_SPAN_NO_CONTRACT_CHECKING
+#endif
+
 #ifndef TCB_SPAN_NO_EXCEPTIONS
 #include <cstdio>
 #include <stdexcept>
@@ -169,7 +174,8 @@ using byte = unsigned char;
 #define TCB_SPAN_NODISCARD
 #endif
 
-TCB_SPAN_INLINE_VAR constexpr std::size_t dynamic_extent = -1;
+TCB_SPAN_INLINE_VAR constexpr std::size_t dynamic_extent =
+    static_cast<std::size_t>(-1);
 
 template <typename ElementType, std::size_t Extent = dynamic_extent>
 class span;
@@ -178,7 +184,7 @@ namespace detail {
 
 template <typename E, std::size_t S>
 struct span_storage {
-  TCB_SPAN_CONSTEXPR span_storage() noexcept = default;
+  constexpr span_storage() noexcept = default;
 
   TCB_SPAN_CONSTEXPR span_storage(E* ptr, std::size_t /*unused*/) noexcept
       : ptr(ptr) {}
@@ -189,7 +195,7 @@ struct span_storage {
 
 template <typename E>
 struct span_storage<E, dynamic_extent> {
-  TCB_SPAN_CONSTEXPR span_storage() noexcept = default;
+  constexpr span_storage() noexcept = default;
 
   TCB_SPAN_CONSTEXPR span_storage(E* ptr, std::size_t size) noexcept
       : ptr(ptr), size(size) {}
@@ -385,7 +391,7 @@ class span {
   TCB_SPAN_CONSTEXPR span(const Container& cont)
       : storage_(detail::data(cont), detail::size(cont)) {}
 
-  TCB_SPAN_CONSTEXPR span(const span& other) noexcept = default;
+  constexpr span(const span& other) noexcept = default;
 
   template <typename OtherElementType, std::size_t OtherExtent,
             typename std::enable_if<
@@ -399,8 +405,7 @@ class span {
 
   ~span() noexcept = default;
 
-  TCB_SPAN_CONSTEXPR_ASSIGN span& operator=(const span& other) noexcept =
-      default;
+  constexpr span& operator=(const span& other) noexcept = default;
 
   // [span.sub], span subviews
   template <std::size_t Count>