ensure that when moving environments the context position is updated

[o-]

if we move an environment over a PushContext we need to update its
context offset field, or it will register itself to the wrong context.
E.g.

e1 = mkenv context 1
     PushContext ... e2
     Checkpoint ....

if we move e1 into the checkpoint, then the context must be increased
to 2, otherwise it will override the environment e2 in the inlined
context.

[o-]

Here are some stats for your PR:

• WARNING: the longest CI job test_release_2 took 2.0h
• are-we-fast-r suite regressed by 0.88
• Mandelbrot regressed by 0.69
• Bounce_nonames_simple regressed by 0.88
• Bounce_nonames regressed by 0.92
• fannkuchredux regressed by 0.84
• reversecomplement_naive regressed by 0.92
• matrixFor regressed by 0.95
• Overall benchmarks regressed by 0.99

Please find your performance results at https://rir-benchmarks.prl.fit.cvut.cz/diff?job_ids[]=1371658575&job_ids[]=1370735306&selection=all

[o-]

Here are some stats for your PR:

• WARNING: the longest CI job test_release_1 took 2.17h
• are-we-fast-r suite regressed by 0.95
• Mandelbrot regressed by 0.88
• Bounce_nonames regressed by 0.92
• Bounce_nonames_simple regressed by 0.94
• emptyFor regressed by 0.95
• Overall benchmarks regressed by 1.0

Please find your performance results at https://rir-benchmarks.prl.fit.cvut.cz/diff?job_ids[]=1372008748&job_ids[]=1370735306&selection=all

[o-]

Here are some stats for your PR:

• WARNING: the longest CI job test_big_inline took 2.36h
• Overall benchmarks improved by 1.0

Please find your performance results at https://rir-benchmarks.prl.fit.cvut.cz/diff?job_ids[]=1372987311&job_ids[]=1371671945&selection=all