Implicitly coerce to string left operand of =~ and !=~ operators

[divtiply]

To prevent (IMO very non-descriptive) error for expressions like this

$ echo x=123 | mlr filter '$x =~ "^[0-9]+$"'
Expression does not evaluate to boolean: got MT_ERROR.

I know I can coerce explicitly, but I see no reason for this

$ echo x=123 | mlr filter 'string($x) =~ "^[0-9]+$"'
x=123

[divtiply]

Intended miller way is

$ echo x=123 | mlr filter -S '$x =~ "^[0-9]+$"'
x=123

See issue #150 and FAQ

[johnkerl]

In the spirit of #151 this will be handled better in the future, without need for -S.

[johnkerl]

@divtiply this already works in Miller 6 -- sorry I didn't close this out sooner! :)

$ mlr --version
Miller v6.0.0-dev

$ echo x=123 | mlr filter -S '$x =~ "^[0-9]+$"'
x=123

[aborruso]

thank you very much @johnkerl