tour: clear up source/destination slice confusion

[crisman]

In "tour/moretypes/10" ("Slice defaults") the phrase "the length of the slice for the high bound" has generated more than a dozen issues, mostly wanting to replace "slice" with "array". The confusion is that when slicing, the word "slice" is not well defined we will have an output slice (result of the slice expression) and may well have a input slice (e.g. in the example code in "slice-bounds.go"). The spec uses the phrase "length of the sliced operand" for the high bound.

Change wording of slice default upper bound to phrase used in "ref/spec" and reword the slide text to point out what the "sliced operands" are in the slide and program examples.

Fixes golang/tour#1386 Fixes golang/tour#879 Fixes golang/tour#855 Fixes golang/tour#821 Fixes golang/tour#679 Fixes golang/tour#448 Fixes golang/tour#182

See golang/tour#182 for context and links to many of the issues.

[gopherbot]

This PR (HEAD: c65e21bd4d22578efb858899fe92a6b128aac284) has been imported to Gerrit for code review.

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[crisman]

Also fixes golang/tour#1337

[crisman]

Also fixes golang/tour#1483