feat(schema): rework schema diffing

[B4nan]

This commit introduces new schema diffing capabilitites. Instead of shared code for
creating and altering tables, we now build schema objects from entities and existing
schema, and compare those (this will later allow to create down migrations too).

• Better mapping - adds mapped types to abstract most of the column types (e.g. { type: t.decimal })
• FK diffing
• Proper index diffing (before we compared just names)
• Custom index expressions
• Comment diffing
• Column length diffing (e.g. numeric(10,2) or varchar(100))
• Changing PK types
• Schena/namespace diffing (posgtres only)

Closes #1486
Closes #1518
Closes #579
Closes #1559
Closes #1602
Closes #1480
Closes #1687

[lgtm-com]

This pull request introduces 6 alerts when merging 63ee3c0 into cb5b25c - view on LGTM.com

new alerts:

• 5 for Unused variable, import, function or class
• 1 for Unused loop iteration variable

[lgtm-com]

This pull request introduces 3 alerts and fixes 1 when merging f2bc957 into 54ca87e - view on LGTM.com

new alerts:

• 3 for Polynomial regular expression used on uncontrolled data

fixed alerts:

• 1 for Polynomial regular expression used on uncontrolled data