[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[dash] Subshells inherit function definitions

Pass function definitions to subshells. To accomplish this,
instead of passing a single command to the subshell we pass a
list of commands, with the function definitions as the first
items in the list.

Change-Id: I0f9dd51e6aaa6bbcfac4acbb83003900fd6d8a72

[dash] Implement foreground subshells

Dash can now execute AST fragments in subprocesses, which means commands
like the following work:

$ (echo 42)
42

Change-Id: I66a994f9de9d3541d1c830c0db75ce09f3196e9f

[dash] Add dash to the build

This patch integrates dash with the build system, including
pre-generating source files.

Change-Id: If19d0be0c030c6c194cb7bf66a26be48a39c0847