Issues with 0.9 and earlier string interpolation

[Licenser]

Problem

There is an issue with string interpolation that does fail to create some strings sensibly. In addition, having a { and } as the last element of a string fails

# this fails
"\}"
# this fails too in different ways
"\{\}"

Steps

1. Create a string with an escaped { in it but no interpolation
2. Create a string with an escaped { in it but with interpolation
3. The escaped { on both is handled different

Possible Solution(s)

Implement the string interpolation RFC tremor-rs/tremor-rfcs#34