Remove ConstToken

[cgewecke]

Solc is ok with this:

uint const = 0;

Solidity-parser is throwing:

SyntaxError: Expected "!=", "%", "%=", "&", "&&", "&=", "(", "*", "*=", "+", "++", "+=", "-", "--", "-=", ".", "/", "/*", "/=", ";", "<", "<<", "<<=", "<=", "=", "==", ">", ">=", ">>", ">>=", ">>>", "?", "[", "^", "^=", "constant", "in", "internal", "memory", "private", "public", "storage", "|", "|=", "||", comment, end of line, or whitespace but "c" found. Line: 303, Column: 8

• const doesn't appear on the reserved keywords list
• can be seen in the wild as a variable name in the consensys/gnosis-contracts here

This PR

• Removes two references to ConstToken
• Adds a unit test that uses the word const in a non-token way. (Sorry)

[federicobond]

LGTM