Ensure consistent naming of predicates in std and core

[boggle]

We discussed this on IRC and decided that names of simple boolean predicates in std and core should start with "is_"
or similarly be expressed using a "small question word"

The notable exception are generally established predicate names like "lt", "ge", etc.

This needs to be checked and materialized with some amount of bikeshedding

[kud1ing]

Is this decision also kept in a coding-style document?

[boggle]

There is one? If so, it should go there

[kud1ing]

Now there is: https://github.com/graydon/rust/wiki/Core-styleguide

[brson]

I'm not motivated to look into this for 0.2