Add basic text benchmarking infrastructure

This adds a new bench command to the interactive shell initially used
for fuzzing with AFL. The syntax is:

 > b op pos [count]

where op is either:

 i (insert)
 d (delete)
 r (replace)
 m (set/get mark)

and pos is one of:

 ^ (start)
 | (middle)
 $ (end)
 % (random)
 - (consecutively from end to start)
 + (consecutively from start to end)
 ~ (stripes with fixed distance)

Hence the following would perform 100 insertions at random positions:

 > b i % 100

Note however, that the used pseudo-random number generator is currently
not seeded, meaning multiple execution will start with the same state,
making them comparable.

Timing is currently performed using monotonic clock_gettime(2).

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