bn.js: Breaking changes for the next major release

(Taking the discussion from #112 to here)

Proposed changes:

• Make .modn() return a BN (#112)
• Renaming .strip() to ._strip() is a breaking change too (#105)
• Maybe reviewing the constructor method(s) could lead to such a change (see #90 & #91)
• Rename .andln() to .andn() (see the README). Perhaps fold the functionality into toArrayLike and make .andn() safe in terms of working up to 53 bits
• Maybe at that stage two’s complement could be made more internal (part of constructor and toString) (see #73)
• Split out the extended functions (saving destroyed bits) off iusrhn() into a specific shift right version, because it is only used in MPrime.split() and is complex.
• Remove internal functions from the BN context (good candidates are: _countBits, _zeroBits, etc. Others could be moved too with passing a self variable instead of using this, however that might have a speed penalty or increase?)
• Perhaps rethink naming/functionality of the following bitwise methods: setn (should be renamed isetn as it is in-place), testn, maskn, bincn