Commit 8c71635ca3 moved the creation to the start
of pipelines; the approach works for simplistic cases like "x = 1 | nop", but
fails in more complex cases, such as "nop (x = 1) | nop".
The correct place to hoist variable creations is the lexical scope, and this
commit implements this approach.
This fixes#623.
The implementations are mostly copied from the builtin joins, replaces and
splits, with small changes to the error behavior in the join function.
The versions in the builtin module are now deprecated.
This avoids race conditions of accessing the local scope. The test case
"x = 1", "put $x | y = (all)" used to contain a race condition but no longer
does.
This addresses #73.
