frontend/ refactor: separate responsabilities to Builder, Compiler and API interfaces

[gbotrel]

This PR builds on #270 . (see description for hint changes).

** API Breaking change **

Fixes #245 ; using options 4. --> frontend.Compile changed from :
frontend.Compile(curveID ecc.ID, backendID backend.ID, circuit Circuit, opts ...CompileOption) to frontend.Compile(curveID ecc.ID, newCompiler frontend.NewCompiler, circuit Circuit, opts ...CompileOption)

** Internal changes **

1. Adds new frontend.Compiler interface, which exposes useful operations to gadget developers that do not fit in frontend.API . For example, to create a std/math/bits package, one needs to have access to MarkBoolean and IsBoolean apis from the constraint system compiler. Naming wise, end-users only see the term Compiler . (not Builders, etc).
2. compiled.Variable is no more. It used to encapsulate a compiled.LinearExpression and a IsBoolean bool value. Replaced with compiled.LinearExpression, IsBoolean is managed through a map.
3. frontend.WithCapacity was broken and compile options were not passed through constraint systems compilers
4. IsConstant removed in favor of ConstantValue (reduces useless big.Int allocations)
5. fix incorrect handling of nbBits == 1 in api.ToBinary