Problem:
- Many constructors are defined as `{}` rather than using the ` =
default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
instead of requiring the caller to default construct. This violates
the C++ Core Guidelines suggestion to declare single-argument
constructors explicit
(https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).
Solution:
- Change default constructors to use the compiler-provided default
constructor.
- Remove implicit conversion operators from `nullptr_t` and change
usage to enforce type consistency without conversion.
- Store history entries as (timestamp)::(entry)\n\n
- Merge the entries together when saving to avoid loss of history
entries
To ideally make having two concurrently open shells
(or `js` repls or whatever) not overwrite each others' history entries.
This fixes an issue that shows up as a nice crash when "^R<enter>^C",
which is actually the event loop trying to call into a deleted object
(the search editor).
This also makes the editor clean as many lines as the searching took,
for instance, in the case of <C-r><C-c>ls<tab>, two lines should be
cleaned, not just one.
Fixes#3413.
Makes C-c print "^C" and continue prompting on a new line.
Also fixes a problem where an interrupted get_line() would need more
read()'s than required to update the display.
Prior to this, no keybinding were installed on the search editor, and so
editing wasn't really possible.
Also fixes <C-r> making infinite search editors.
