Add support for unicode strings

[aquasync]

I was trying to read a cell value in Excel that contained unicode data. I found that strings with non-ascii values seem to just return a blank string:

excel$ActiveWorkbook()$ActiveSheet()$Range('A1')$Value()
[1] ""
Encoding(s)
[1] "unknown"

This pull request changes FromBstr/AsBstr to use UTF8, so that this now works as expected:

(s<-excel$ActiveWorkbook()$ActiveSheet()$Range('A1')$Value())
[1] "1234³"
Encoding(s)
[1] "UTF-8"

An alternative workaround that might be applicable for others is to use win32com.client via reticulate, but I didn't want to require a python installation:

library(reticulate)
excel2 = import('win32com.client')$Dispatch('excel.application')
excel2$ActiveWorkbook$ActiveSheet$Range('A1')$Value
[1] "1234³"

I've also verified that writing unicode values works now with these changes also.

Note that there are still a bunch of places where we use non-encoding aware conversions, primarily around dispatch (method/property/type names etc), which I presume is fine. The focus here was on errors and values.

Let me know if there's some changes you'd want to see for this to be accepted. Thanks!