Ability to change the initial filter for completion

[roosephu]

Currently, elvish builds an initial candidate list before completion and uses the interactive Completion mode to refine the list. For example, for the input of put $edit:re<Tab>, elvish builds a candidate list [redraw, return-eof, return-line, replace-input] and the user might use an additional filter (e.g., line) to find the candidate.

The proposal here is that we unify the two filtering processes (initial candidate list building and further refining). With this proposal, in the previous example, elvish builds a list containing all possible candidates under $edit: and sets the initial filter to re in the interactive Completion mode. Similar behavior is also expected for ls /tmp/fo.

The advantage is that it's now possible to modify the initial filter after pressing Tab. In the example, if re is too restrictive, the user must close the Completion mode first, change the filter and press Tab again before the proposal. It also sounds more intuitive to me.

Unresolved questions:

1. How should smart-start work with this proposal?
2. Currently, the initial candidate list is filtered by prefix matching by default. However, the Completion mode uses substring matching, so the new behavior is inconsistent with the old one.

Currently, it's implemented by #1463.

@xiaq @krader1961