FIND/PART not working after skipping the series

[hostilefork]

This bug existed in Red as well, but was patched. Copying from that issue:

>> str: "1.1.1"
== "1.1.1"
>> find/part str "1." 2
== "1.1.1"
>> str: skip str 2
== "1.1"
>> find/part str "1." 2
== none
>> find str "1."
== "1.1"

[hostilefork]

This problem comes from a mistake in interpreting the return result of the helpers for decoding Partial()/Partial1(). They return a length in characters offset from the VAL_INDEX() of the series, not an absolute tail position:

https://github.com/rebol/rebol/blob/25033f897b2bd466068d7663563cd3ff64740b94/src/core/t-string.c#L516

To get the tail, the updated VAL_INDEX() (which may be modified during Partial()) should be added to the length returned.

This commit does a general cleanup to address this and other issues, and run more of the code through the same path...unifying Partial() and Partial1() and giving them better names. For instance, a Part_Len vs. Part_Tail specialization addresses the distinction for this issue:

metaeducation/ren-c@07e32b8