Add licensing information and update all source code files to include SPDX license headers

[zerosnacks]

• Create a THIRD_PARTY_LICENSES file documenting the third party licenses
• Create a LICENSE file documenting the general license (license headers from each file overrule the general rule)
• Add SPDX license headers to each source file
• Clarify, evaluate and update the license files where necessary

[zerosnacks]

PR in progress: #51

[zerosnacks]

https://github.com/curvefi/curve-contract/blob/master/LICENSE could serve as a source of inspiration for our general LICENSE