defer() and all local_*() functions now work when run inside of
a .Rmd. The deferred expressions are executed when knitr exits.

defer() and local_ functions now work within source().
The deferred expressions are executed when source() exits.

with_() and local_() gain a get argument. Supply a getter
function to create with and local functions that are robust to
early exits.

When supplied, this restoration pattern is used:

old <- get()
on.exit(set(old))
set(new)
action()

Instead of:

old <- set(new)
on.exit(set(old))
action()

This ensures proper restoration of the old state when an early exit
occurs during set() (for instance when a deprecation warning is
caught, see #191).

These with_ and local_ functions are now robust to early exits (see next bullet):

• _locale()
• _envvar()
• _libpaths()
• _options()
• _par()
• _path()
• _seed()

with_namespace() and local_namespace() now pass warn.conflicts
to attach() (@kyleam, #185).

local_rng_version() and local_seed() no longer warn when
restoring sample.kind to "Rounding" (#167).

with_seed() now preserves the current values of RNGkind() (#167).

with_collate() is no longer affected by the LC_COLLATE
environment variable set to "C" (#179).

Local evaluations in the globalenv() (as opposed to top-level
ones) are now unwound in the same way as regular environments.

local_tempfile() gains a lines argument so, if desired, you can pre-fill
the temporary file with some data.