v1.5.0: A milestone release - signed arithmetic.

This release brings us:

• basic signed arithmetic - sgadd, sgsub, sgmul, sgdiv, sgmod, sneg (signed negation), abs, sgn (signum).
• faster builds, better tests.
• asm2bf passes the magical amount of 35 000 lines of code, 130 instructions and 900 commits.
• enhanced error reporting.
• minor bugfixes
• cbegin and cend blocks for conditional instructions!

Roadmap for future versions:

• conditional variants
• comparisons
• conditional pipeline primers (1st and 2nd kind) for the comparisons.
• signed floating point arithmetic.
• chebyshev coefficients & trigonometric function approximation.
• floating point & constant square root.
• floating point approximation (calculate Nth decimal place).