Now that that 'peg' itself is registered as a tool in go.mod we can call
it directly via 'go tool peg' in '//go:generate' lines (in grammars):
peg will be automatically built (and cached) as necessary.

* Simplify generate-grammars.bash using Go package patterns.
* Forward flags to 'go generate'

Example:
$ ./generate-grammars.bash -n
go tool peg -switch -inline c.peg
go tool peg -switch -inline calculator.peg
go tool peg -switch -inline calculator.peg
go tool peg -switch -inline fexl.peg
go tool peg -switch -inline java_1_7.peg
go tool peg -switch -inline long.peg

Use 'go tool peg' (instead of 'go build && ./peg') to run peg for
regenerating peg.peg.go.