Fix typographical and grammatical errors and inconsistencies.

The initial list of hits was generated with the codespell command
below, and each hit was evaluated and fixed manually while taking
context into consideration.

    DIRS="kernel/ frontends/ backends/ passes/ techlibs/"
    DIRS="${DIRS} libs/ezsat/ libs/subcircuit"
    codespell $DIRS -S *.o -L upto,iff,thru,synopsys,uint

More hits were found by looking through comments and strings manually.

kernel/log.h

@@ -195,7 +195,7 @@ struct PerformanceTimer
 		t += 1000000000ULL * (int64_t) rusage.ru_stime.tv_sec + (int64_t) rusage.ru_stime.tv_usec * 1000ULL;
 		return t;
 #  else
-#    error Dont know how to measure per-process CPU time. Need alternative method (times()/clocks()/gettimeofday()?).
+#    error "Don't know how to measure per-process CPU time. Need alternative method (times()/clocks()/gettimeofday()?)."
 #  endif
 	}