We'll use this to call the Windows ReplaceFile API for safe file
replacement when updating credentials.tf.json in "terraform login" and
"terraform logout".
This brings in a bugfix for analyzing variables inside relative traversal
expressions in HCL, and a cosmetic bugfix in cty for GoString of
cty.NullVal(cty.DynamicPseudoType).
This also updates some other packages, as a result of running "go get -u".
This includes a bugfix to the cty/msgpack package to ensure correct
decoding of unknown and null values.
This also includes updates to cty's dependencies.
Martin Atkins