log: general-purpose logging system

The log module provides a simple interface for application logging. The
[[logger]] interface provides an abstraction that users may implement for custom
logging logic. We provide a simple logger implementation that prefixes each line
with the current timestamp; to initialize such a logger see [[new]].

A global logger is also provided for simple applications to use, [[global]],
which is an instance of the standard logger that writes to [[os::stderr]] by
default. The user may configure a new global logger via [[setlogger]].
