Printing: rename bound variables if they occur in the context

[fblanqui]

• print: add new argument providing the names to avoid for bound variables
• proof: add get_names to avoid for bound variables when printing hypotheses and goals + some renamings in printing functions
• pure: avoid context variable names when printing hypotheses and goals
• query: avoid context variable names when printing query results
• tactic, unif: avoid context variable names when printing errors
• extra: reimplement (and fix) get_safe_prefix, and add unit tests
• ctxt: add names and fresh
• env: add names
• tactic: move gen_valid_idopts to env.ml

TODO:

• Lplib.Extra.root_and_index: handle leading sequences of zeros
• reset idset when printing an error message
• pass idset as an argument to printing functions ?
• in tactic, etc. remove directs settings of idset and use printing functions taking an env or a ctxt as argument ?
• use a more efficient data structure than StrSet.t to test/generate names
• fix Renaming bound variables equal to a symbol name #344