Refactor bootstrap.bash to improve readability of the bootstrap steps

[dolmen]

Refactor bootstrap.bash:

1. Use "go run" to execute each step: this allows to build, run, and cleanup the binary in a single line
2. Make each step a single command with an explicit list of source files which are part of the build, and aligned input and output files: this improves readibility
3. Cleanup intermediate files in case of success.
4. As the previous build script was leaving artefacts, add cleanup of those obsolete artefacts as they are no longer relevant and could be misleading.

Extract of the final script:

go run ../../bootstrap
go run -tags bootstrap  main.go bootstrap.peg.go     < bootstrap.peg     > peg1.peg.go
go run -tags bootstrap  main.go peg1.peg.go          < peg.bootstrap.peg > peg2.peg.go
go run -tags bootstrap  main.go peg2.peg.go          < ../../peg.peg     > peg3.peg.go
go run -tags bootstrap  main.go peg3.peg.go          < ../../peg.peg     > peg-bootstrap.peg.go
go run -tags bootstrap  main.go peg-bootstrap.peg.go < ../../peg.peg     > ../../peg.peg.go

Note: next refactor step should about refactoring ./bootstrap/main.go to make its output more explicit.