Add :resetblocks / :rb to REPL

[johnkerl]

This is for #918

$ rlwrap mlr repl
Miller 6.0.0-dev REPL for darwin/amd64/go1.17
Docs: https://miller.readthedocs.io
Type ':h' or ':help' for online help; ':q' or ':quit' to quit.

[mlr] :h repl-list
:l or :load
:o or :open
:reopen
:r or :read
:w or :write
:rw
:c or :context
:s or :skip
:p or :process
:w or :>
:w or :>>
:b or :begin
:m or :main
:e or :end
:astprint
:blocks
:rb or :resetblocks
:q or :quit
:h or :help

[mlr] :h :rb
:rb
:resetblocks with no arguments.
Clears out all begin, main, and end blocks that have been loaded.

[mlr] :blocks
#begin 0
#main  0
#end   0

[mlr] begin { print "Hello" }
[mlr] end { print "world!" }
[mlr] :b
Hello
[mlr] :e
world!
[mlr] :blocks
#begin 1
#main  0
#end   1

[mlr] :rb

[mlr] :b
[mlr] :e
[mlr] :blocks
#begin 0
#main  0
#end   0
[mlr]

[vapniks]

Would be nice if you could also reset begin, main & end blocks independently, e.g. by passing an arg to :rb

[johnkerl]

@vapniks was thinking same, glad to hear demand for it :D