unpack: conditionally use a file mapping to write files

[joaocgreis]

Use a file mapping to write files up to 512KB.

The limit where using a file mapping stops being an advantage varies from machine to machine. 512KB is a reasonable value to use here, close to the lower bound, to avoid tar becoming slower in some machines.

This is an alternative to #227 and also makes npm/pacote#8 unnecessary.

cc @isaacs

[isaacs]

Published to 5.0.1, and extracted to a separate module in 97f813a so that it could get better test coverage. I prefer to avoid using /* istanbul ignore next */ unless absolutely necessary (ie, throw new Error('impossible') in code paths that we expect to be unreachable).

[joaocgreis]

Thanks @isaacs!