assert: make tHelper a type alias

[dolmen]

Summary

The tHelper interface is defined only for internal usage. We don't need a "hard" type for it as we don't define methods on it. Let's make is just a alias to the anonymous interface it represents.

Note: we are already using type aliases elswhere, and type aliases were introduced a long time ago in Go 1.9.

Changes

• assert: tHelper becomes a type alias
• require: tHelper becomes a type alias

Motivation

The tHelper symbol is not needed at runtime. This should reduce resource usage (yes, negligible).

Related issues

• 1422 is also related to the interface { Helper() } type, but not dependent

[dolmen]

@brackendawson There is no significant performance improvement (or degradation) expected. The resource usage I claim is just the absence of the type definition in the final binary, so a smaller binary.

Compare the output of this command on this branch vs master:

$ (cd assert ; go test -c && strings assert.test | grep BoolAssertionFunc)