Replace 1/sqrt(2*pi) calculations with constants

[sunsided]

This changes some function calls to avoid divisions and replace them with constants.

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This PR also includes #27 for use of black_box in benchmarks. Compared to that I get slighly reduced mean durations with improved error bounds:

running 3 tests
test bench_all_price          ... bench:          57.02 ns/iter (+/- 3.87)
test bench_all_price_no_cache ... bench:         235.70 ns/iter (+/- 64.07)
test bench_call_price         ... bench:          38.98 ns/iter (+/- 6.24)

Like in #27, the benchmark test will also fail here due to the changed duration baselines.

[coveralls]

Pull Request Test Coverage Report for Build 9243534858

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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Details

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 79.967%

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Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	1	79.97%

Totals
Change from base Build 4354937295:	-0.6%
Covered Lines:	407
Relevant Lines:	446

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💛 - Coveralls

[danielhstahl]

Thank you, can you bump https://github.com/danielhstahl/black_scholes_rust/blob/master/Cargo.toml#L3 to 10.2?

[sunsided]

Done!