Robj to R choice

[sorhawell]

This PR introduce internal conversion robj_to_rchoice() which takes any Robj and if char vec return first string value, other wise returning an Error explaining why this is not valid choice type.

The conversion is furthermore used to impl robj_to!(JoinType, how)? and robj_to!(ClosedWindow, closed_window)?. No match.arg(arg = ,choices = ,several.ok = ) will be needed on R side.

ok example
pl$LazyFrame()$join(pl$LazyFrame(), how = c("inner", "left", "outer", "semi", "anti", "cross"), on="tofu")

new err examples

> pl$LazyFrame()$join(pl$LazyFrame(), how = list(42), on="tofu")
Error: Execution halted with the following contexts
0: In R: in $join():
0: During function call [pl$LazyFrame()$join(pl$LazyFrame(), how = list(42), on = "tofu")]
1: The argument [how] caused an error
2: Got value [Rvalue: list!(42.0), Rsexp: List, Rclass: ["list"]]
3: Not a valid R choice because input is not a character vector

> pl$LazyFrame()$join(pl$LazyFrame(), how = "bob", on="tofu")
Error: Execution halted with the following contexts
0: In R: in $join():
0: During function call [pl$LazyFrame()$join(pl$LazyFrame(), how = "bob", on = "tofu")]
1: The argument [how] caused an error
2: Got value [JoinType choice ['bob'] is not any of 'cross', 'inner', 'left', 'outer', 'semi', 'anti']

[etiennebacher]

LGTM, thanks @sorhawell !