[m-rev.] for post-commit review: speed up the term parser

[Zoltan Somogyi]

This diff speeds up the compiler by about 6%. I haven't seen
such a big speedup in a long time. The raw timing data follows.

Zoltan.

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SPEED.parser.feb18
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mercury_compile.1 17.707u 0.494s 0:18.22 99.8%	0+0k 0+31744io 0pf+0w
mercury_compile.2 16.451u 0.111s 0:16.59 99.8%	0+0k 8+31584io 0pf+0w
mercury_compile.1 17.270u 0.095s 0:17.40 99.7%	0+0k 8+31584io 0pf+0w
mercury_compile.2 16.321u 0.083s 0:16.49 99.4%	0+0k 16+31584io 0pf+0w
mercury_compile.1 17.260u 0.107s 0:17.41 99.7%	0+0k 24+31584io 0pf+0w
mercury_compile.2 16.340u 0.095s 0:16.54 99.3%	0+0k 40+31584io 0pf+0w
mercury_compile.1 17.575u 0.103s 0:17.69 99.8%	0+0k 0+31584io 0pf+0w
mercury_compile.2 16.571u 0.123s 0:16.71 99.8%	0+0k 0+31584io 0pf+0w
mercury_compile.1 17.809u 0.103s 0:17.93 99.8%	0+0k 0+31584io 0pf+0w
mercury_compile.2 16.661u 0.111s 0:16.79 99.8%	0+0k 0+31584io 0pf+0w
mercury_compile.1 18.134u 0.107s 0:18.28 99.7%	0+0k 8+31584io 0pf+0w
mercury_compile.2 17.146u 0.067s 0:17.24 99.7%	0+0k 16+31584io 0pf+0w
mercury_compile.1 18.024u 0.087s 0:18.14 99.7%	0+0k 8+31584io 0pf+0w
mercury_compile.2 16.952u 0.099s 0:17.11 99.5%	0+0k 24+31584io 0pf+0w
mercury_compile.1 18.577u 0.099s 0:18.71 99.7%	0+0k 8+31584io 0pf+0w
mercury_compile.2 16.760u 0.067s 0:16.85 99.8%	0+0k 0+31584io 0pf+0w
mercury_compile.1 17.953u 0.079s 0:18.09 99.6%	0+0k 16+31584io 0pf+0w
mercury_compile.2 17.075u 0.079s 0:17.17 99.8%	0+0k 0+31584io 0pf+0w
mercury_compile.1 17.997u 0.103s 0:18.13 99.7%	0+0k 8+31584io 0pf+0w
mercury_compile.2 17.034u 0.103s 0:17.15 99.8%	0+0k 0+31584io 0pf+0w
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SPEEDSUM.parser.feb18
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mercury_compile.1 average of 10 with ignore=1     17.81
mercury_compile.2 average of 10 with ignore=1     16.73
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