Swap out github.com/matryer/is with internal implementation

[matthewmueller]

https://github.com/matryer/is is wonderfully simple, but almost too simple. I'm finding myself needing a .Contains equivalent more times than not. The best way to do a contains currently is:

is.True(strings.Contains(actual, expect))

This isn't great because while it tells you an error occurred on that line, it doesn't tell you the values of actual and expect.

To summarize,

What I love:

• It's simple API
• The formatting and line numbers.
• How it pulls in the actual function call

What I don't really like:

• It's missing a "Contains"
• Use of comments for modifying the error message
• Unnecessary indentation (issue)

Proposal

• Pull out the caller and formatting functions from https://github.com/matryer/is
• Remove commenting support
• Create an internal is package with is.In(actual, expect) (aka .Contains())
• Add more functions as needed