jiri: Add helper functions to envvar to manipulate token lists.
jiri: Add helper functions to envvar to manipulate token lists.

Renames {Prepend,Append}UsingSeparator to
{Prepend,Append}UniqueToken, and changes the semantics to also
filter away duplicate tokens.  The previous functions didn't seem
to provide much value over manually calling {Split,Join}Tokens.
The new functions provide a bit more functionalty wrapped into a
one-line call.

Also added UniqueTokens and FilterTokens, which help manipulate
slices of tokens.

Change-Id: Idf0f5a1f2cf88acf1e320326a328e3f2ec83fb55
MultiPart: 1/2

vanadium/go.jiri,vanadium/go.jiri