Thm.dest_abs now takes an additional argument.

Moved rewriting functions from Thm to MetaSimplifier.

- Moved rewriting functions to meta_simplifier.ML - dest_abs now also takes an optional variable name as an argument - beta_conversion takes additional flag as an argument. Fully reduces the term if set to true Some tuning: - tuned fix_shyps in instantiate, implies_intr, implies_elim, reflexive, transitive, beta_conversion, abstract_rule - combination: chktypes derives types of f and t from type of == instead of using fastype_of New primitives: - eta_conversion - incr_indexes: increment indexes in theorems - cterm_incr_indexes: increment indexes in cterms - cterm_match, cterm_first_order_match - rename_boundvars