Version 0.7.1: Work on unique_span, minor bug fixes and tweaks

Changes since v0.7.0:

CUDA Graphs

• #532 Now supporting empty nodes as graph::typed_node's.
• #656 When capturing a graph on a CUDA stream - default to capturing on using the global capture mode task
• #657 Offer stand-alone functions for stream capture begin and end tasks (as opposed to only stream_t class methods)

unique_span class changes

Remember: unique_span<T, Deleter> is like a unique_ptr<T, Deleter> but with specified size, and interoperability with std::span/cuda::span.

• #662 unique_span will no longer be constructible from uspans with other deleter classes via an implicit conversion into a span. Also,
• it will no longer be constructible from untyped memory regions.
• #660 unique_span assignement operator bug fixes.
• #652 unique_span is now default-constructible.

Other changes

• #651 Can now access the default stream of a non-primary context via stream_t
• #653 Can now launch in an arbitrary CUDA context without referring to a specific stream (i.e. using the default stream for the context)
• #654 The logic of context_t::create_module() is now the same as for context::module::create()
• #659 The methods context_t::create_stream() and context_t::create_event() are now marked const