Add requireRequestNotNull

[mhickman]

This is inspired directly from kotlin.requireNotNull and meant to simplify usages of requireRequest in code. The intent is before you have code that looks like this

requireRequest(request.parameter != null) { "parameter must not be null" }

// later, safe do because of previous null check
request.parameter!!

becomes

requireRequestNotNull(request.parameter) { "parameter must not be null" }

// later - no need for !!
request.parameter