Unify and fix tasklet.kill(), tasklet.throw() and tasklet.raise_exception()

Originally reported by: Anselm Kruis (Bitbucket: akruis, GitHub: akruis)

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The implementations of all three methods differ in their error handling. And they contain bugs in corner cases, especially if a tasklet has no thread state (various assertion violations).
I created a unified implementation of all three methods.

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• Bitbucket: https://bitbucket.org/stackless-dev/stackless/issue/93

Original comment by Anselm Kruis (Bitbucket: akruis, GitHub: akruis):

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Test cases (still skipped) for 2.7-slp are in 1399a7b0c676.

Original comment by Anselm Kruis (Bitbucket: akruis, GitHub: akruis):

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The fix for 2.7-slp is in pull request #18 (merged).

Original comment by Anselm Kruis (Bitbucket: akruis, GitHub: akruis):

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Fixed:

• 3.3-slp: e3d5e6b2300f
• 3.4-slp: e81d51157952, ff1e833e4ac6
• default-slp: 120f33c1443c, 3b8d3cd58009