Porting code to System.Memory

With PR https://github.com/mono/monodevelop/pull/5570, we now have support for using the new Span/Memory APIs.

What does this entail? Lots of performance niceties.

This guide will go through common patterns that can be ported over to the new APIs with a mapping for each method and how it should be ported.

Porting guidelines

To get a ReadOnlySpan<char> from a string, use AsSpan().

To get a string from a ReadOnlySpan<char>, use ToString().

We currently do not have stackalloc span support until we use C# 7.3, so use "mystring".AsSpan() where you need a local span.

String API	ReadOnlySpan API
==	SequenceEquals
Substring	Slice
Trim*	Trim*
Starts/EndsWith	Starts/EndsWith
Equals	Equals

Note: any StringComparison other than Ordinal/OrdinalIgnoreCase will convert to a string internally, so you won't get any benefit for equality there.

Common patterns

Substring+Trim

Normally, a string substring then a trim would allocate two strings.