Intern constant values after parsing to deduplicate

[frebib]

Duplicate values should use the same pointers in memory

11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c510 const: str("exec0")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c630 const: str("exec0")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100cab0 const: str("exec0")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100ca20 const: str("exists")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc0006b2460 composite: []str
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c7e0 const: str("/tmp/command-output")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c870 const: str("/tmp/command-output")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c990 const: str("/tmp/command-stderr")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c5a0 const: str("/tmp/command-stdout")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c900 const: str("/tmp/command-stdout")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c750 const: str("exec0")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100cbd0 const: str("/bin/bash")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100c6c0 const: str("/tmp/command-stderr")
11:13:35 main.go:75: gapi: lang: funcs: Engine:AddVertex: 0xc00100cb40 const: str("echo this is stdout && echo this is stderr 1>&2")