bench: calc: work on a string instead of a file

The cost of the file layer is large and makes benchmarks too coarse,
as seen for in following example, first with a file, then with a
literal string:

    0. %skeleton "yacc.c" %define parse.lac full
    1. %skeleton "yacc-v1.c" %define nofinal %define parse.lac full
    2. %skeleton "yacc-v2.c" %define nofinal %define parse.lac full
    3. %skeleton "yacc-v3.c" %define nofinal %define parse.lac full
    4. %skeleton "yacc.c"
    5. %skeleton "yacc-v1.c" %define nofinal
    6. %skeleton "yacc-v2.c" %define nofinal
    7. %skeleton "yacc-v3.c" %define nofinal
    --------------------------------------------------
    Benchmark           Time           CPU Iterations
    --------------------------------------------------
    BM_y0           32558 ns      32537 ns      21228
    BM_y1           32400 ns      32369 ns      21233
    BM_y2           33485 ns      33464 ns      20625
    BM_y3           32139 ns      32125 ns      21446
    BM_y4           31343 ns      31329 ns      21747
    BM_y5           31344 ns      31317 ns      22035
    BM_y6           31287 ns      31255 ns      22039
    BM_y7           31387 ns      31373 ns      22178
    --------------------------------------------------
    Benchmark           Time           CPU Iterations
    --------------------------------------------------
    BM_y0           10642 ns      10634 ns      63601
    BM_y1           10657 ns      10654 ns      63625
    BM_y2           10441 ns      10432 ns      65957
    BM_y3           10558 ns      10554 ns      64546
    BM_y4            9521 ns       9516 ns      72011
    BM_y5            9179 ns       9157 ns      75028
    BM_y6            9360 ns       9356 ns      73770
    BM_y7            9365 ns       9359 ns      72609

Of course, at the same time it is less realistic: most users read
files rather that strings, so it might lead to us to pay attention to
costs most people don't see.

* etc/bench.pl.in (&calc_input): Output into a file given as argument.
Output in C syntax.
(&generate_grammar_calc): Use it.
Simplify the grammar: remove operators we don't care about.
Rewrite the scanner to work on a char* instead of a FILE*.

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