dolphin: Add function signatures

[kyleconroy]

Slowly add function signatures for the MySQL standard library. Since sqlc doesn't yet contemplate types correctly, it's more important to get the return value and number of arguments correct.

Functions and operators

• https://dev.mysql.com/doc/refman/8.0/en/mathematical-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/string-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html
• https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/json-function-reference.html
• https://dev.mysql.com/doc/refman/8.0/en/encryption-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/miscellaneous-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/information-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/window-function-descriptions.html
• https://dev.mysql.com/doc/refman/8.0/en/performance-schema-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/locking-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/regexp.html
• https://dev.mysql.com/doc/refman/8.0/en/spatial-function-reference.html
• https://dev.mysql.com/doc/refman/8.0/en/gtid-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/xml-functions.html

Fixes #657