use aligned flex tables

~2% speedup on parsing without eval, less (but still significant) on
system eval. having flex generate faster parsers leads to very strange
misparses. maybe re2c is worth investigating.

before:

  Time (mean ± σ):      4.260 s ±  0.003 s    [User: 3.754 s, System: 0.505 s]
  Range (min … max):    4.257 s …  4.266 s    10 runs

after:

  Time (mean ± σ):      4.231 s ±  0.004 s    [User: 3.725 s, System: 0.504 s]
  Range (min … max):    4.226 s …  4.240 s    10 runs

src/libexpr/lexer.l

@@ -1,4 +1,5 @@
 %option reentrant bison-bridge bison-locations
+%option align
 %option noyywrap
 %option never-interactive
 %option stack