combine functions behind to/from-c/tsv commands

fixes #969, admittedly without a --delimiter alias

moves from_structured_data.rs to from_delimited_data.rs to better
identify its scope and adds to_delimited_data.rs. Now csv and tsv both
use the same code, tsv passes in a fixed '\t' argument where csv passes
in the value of --separator

docs/commands/from-csv.md

@@ -92,6 +92,10 @@ error: Expected a string from pipeline
 ━━━┷━━━━━━━━━━━┷━━━━━━━━━┷━━━━━━
 ```
 
+The string '\t' can be used to separate on tabs. Note that this is the same as using the from-tsv command.
+
+Newlines '\n' are not acceptable separators.
+
 Note that separators are currently provided as strings and need to be wrapped in quotes.
 
 ```shell