Also replace (*Frame).OutputChan with (*Frame).ValueOutput, which returns a
small interface for writing to the value output that is also aware when the
reader is gone.
This also replaces the slightly awkward "arguments here" reason with
"argument count" as the "what" for a typical errs.ArityMismatch
exception. It also reformats most of the constructors so that the "what"
is on the same line. This makes `grep errs.ArityMismatch **.go` more
useful as a result.
- Make Evaler mostly thread-safe. The only remaining thread-unsafe part is the
modules field, which is more tricky than other fields.
- Remove the state and evalerScopes type, and move their fields into Evaler.
- Expose valuePrefix via a get method, and change PortsFromFiles to take the
prefix instead of a *Evaler. Also expose a PortsFromStdFiles.
- Make Evaler a normal field of Frame, instead of an embedded field. This makes
access to global states more explicit.
To facilitate using the `^` character for line continuation we need to
remove its use an an exponentiation function. So introduce `math:pow`
and `math:pow10` as alternatatives and mark `^` as deprecated.
Related #989
