fixes #155 slow plonk compile, fixes #163 detects unconstrained inputs

[gbotrel]

• fixes perf: frontend.Compile with PLONK became too slow #155 slow plonk compile
• fixes Ensures frontend.Compile picks up unconstrained inputs #163 detects unconstrained inputs

API change:

• adds frontend.CompileOptions which enables to set initial capacity (as previously) and to ignore compiler error if unconstrained inputs are detected

Internal:

• when splitting linear expression in plonk frontend, do less gcd and less linear expression division.