New Rule: no-implicit-type-coercion

[adidahiya]

I'd like to forbid null checks which use implicit type coercion:

if (nonBooleanValue) {
    ...
}

and instead enforce checks like this:

if (nonBooleanValue != null) {
    ...
}

[adidahiya]

blocked on complete type information, which will be enabled by #680

[adidahiya]

related: #253

[andy-hanson]

strict-boolean-expressions should do this.