Panic during delete

[Panky-codes]

Steps to reproduce the panic:

• Set the degree to be two (b=2)
• Add: a, b, c, d, e, f, g, h, j
• Remove: j,i,h,g

I adapted the delete_works test case with the above scenario.

This results in the following panic:

running 1 test
test btree::tests::delete_works ... Keys [Key("f")] idx 1
thread 'btree::tests::delete_works' panicked at src/btree.rs:334:22:
removal index (is 1) should be < len (is 1)

This is how the tree looks after deleting j,i,h:

Node at offset: 122880
|->Keys: [Key("d")]
|->Children: [Offset(81920), Offset(126976)]
   |  Node at offset: 81920
   |  |->Keys: [Key("b")]
   |  |->Children: [Offset(24576), Offset(49152)]
   |     |  Node at offset: 24576
   |     |  |->Key value pairs: [KeyValuePair { key: "a", value: "a" }, KeyValuePair { key: "b", value: "b" }]
   |     |  Node at offset: 49152
   |     |  |->Key value pairs: [KeyValuePair { key: "c", value: "c" }, KeyValuePair { key: "d", value: "d" }]
   |  Node at offset: 126976
   |  |->Keys: [Key("f")]
   |  |->Children: [Offset(69632), Offset(131072)]
   |     |  Node at offset: 69632
   |     |  |->Key value pairs: [KeyValuePair { key: "e", value: "e" }, KeyValuePair { key: "f", value: "f" }]
   |     |  Node at offset: 131072
   |     |  |->Key value pairs: [KeyValuePair { key: "g", value: "g" }]

When we delete g, we do the following:

• we merge the e, f node with the g node
• delete parent node f which is resulting in panic at btree.rs:334

I think we need to add a condition which checks the length of the node for underflow, before removing the key at btree.rs:334. If it is an underflow, then maybe we need to move the node and its children up?